<?xml version="1.0" encoding="us-ascii"?><InstanceReport xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"><Version>2.4.0.8</Version><ReportLongName>0010 - Disclosure - DERIVATIVE WARRANT LIABILITY</ReportLongName><DisplayLabelColumn>true</DisplayLabelColumn><ShowElementNames>false</ShowElementNames><RoundingOption /><HasEmbeddedReports>false</HasEmbeddedReports><Columns><Column FlagID="0"><Id>1</Id><IsAbstractGroupTitle>false</IsAbstractGroupTitle><LabelSeparator>

</LabelSeparator><CurrencyCode /><FootnoteIndexer /><hasSegments>false</hasSegments><hasScenarios>false</hasScenarios><MCU><KeyName /><CurrencySymbol /><contextRef><ContextID>From2013-01-01to2013-06-30</ContextID><EntitySchema>http://www.sec.gov/CIK</EntitySchema><EntityValue>0001031927</EntityValue><PeriodDisplayName /><PeriodType>duration</PeriodType><PeriodStartDate>2013-01-01T00:00:00</PeriodStartDate><PeriodEndDate>2013-06-30T00:00:00</PeriodEndDate><Segments /><Scenarios /></contextRef><UPS /><CurrencyCode /><OriginalCurrencyCode /></MCU><CurrencySymbol /><Labels><Label Key="CalendarSupplement" Id="0" Label="6 Months Ended" /><Label Key="Calendar" Id="1" Label="Jun. 30, 2013" /></Labels></Column></Columns><Rows><Row FlagID="0"><Id>1</Id><IsAbstractGroupTitle>true</IsAbstractGroupTitle><LabelSeparator>

</LabelSeparator><Level>1</Level><ElementName>ecte_NotesToFinancialStatementsAbstract</ElementName><ElementPrefix>ecte_</ElementPrefix><IsBaseElement>false</IsBaseElement><BalanceType>na</BalanceType><PeriodType>duration</PeriodType><IsReportTitle>false</IsReportTitle><IsSegmentTitle>false</IsSegmentTitle><IsCalendarTitle>false</IsCalendarTitle><IsEquityPrevioslyReportedAsRow>false</IsEquityPrevioslyReportedAsRow><IsEquityAdjustmentRow>false</IsEquityAdjustmentRow><IsBeginningBalance>false</IsBeginningBalance><IsEndingBalance>false</IsEndingBalance><IsReverseSign>false</IsReverseSign><FootnoteIndexer /><Cells><Cell FlagID="0" ContextID="" UnitID=""><Id>1</Id><IsNumeric>false</IsNumeric><IsRatio>false</IsRatio><DisplayZeroAsNone>false</DisplayZeroAsNone><NumericAmount>0</NumericAmount><RoundedNumericAmount>0</RoundedNumericAmount><NonNumbericText /><FootnoteIndexer /><CurrencyCode /><CurrencySymbol /><IsIndependantCurrency>false</IsIndependantCurrency><ShowCurrencySymbol>false</ShowCurrencySymbol><DisplayDateInUSFormat>false</DisplayDateInUSFormat></Cell></Cells><ElementDataType>xbrli:stringItemType</ElementDataType><SimpleDataType>string</SimpleDataType><IsTotalLabel>false</IsTotalLabel><UnitID>0</UnitID><Label>Notes to Financial Statements</Label></Row><Row FlagID="0"><Id>2</Id><IsAbstractGroupTitle>false</IsAbstractGroupTitle><LabelSeparator>

</LabelSeparator><Level>2</Level><ElementName>ecte_DerivativeWarrantLiabilityTextBlock</ElementName><ElementPrefix>ecte_</ElementPrefix><IsBaseElement>false</IsBaseElement><BalanceType>na</BalanceType><PeriodType>duration</PeriodType><IsReportTitle>false</IsReportTitle><IsSegmentTitle>false</IsSegmentTitle><IsCalendarTitle>false</IsCalendarTitle><IsEquityPrevioslyReportedAsRow>false</IsEquityPrevioslyReportedAsRow><IsEquityAdjustmentRow>false</IsEquityAdjustmentRow><IsBeginningBalance>false</IsBeginningBalance><IsEndingBalance>false</IsEndingBalance><IsReverseSign>false</IsReverseSign><FootnoteIndexer /><Cells><Cell FlagID="0" ContextID="From2013-01-01to2013-06-30" UnitID=""><Id>1</Id><IsNumeric>false</IsNumeric><IsRatio>false</IsRatio><DisplayZeroAsNone>false</DisplayZeroAsNone><NumericAmount>0</NumericAmount><RoundedNumericAmount>0</RoundedNumericAmount><NonNumbericText>&lt;p style="font: 10pt/normal Times New Roman, Times, Serif; margin: 0; text-align: justify; text-indent: 9pt"&gt;&amp;#160;At June 30,
2013 and December 31, 2012, the Company had outstanding warrants to purchase 1,050,454 and 1,254,004 shares of its Common Stock,
respectively. Included in these outstanding warrants at June 30, 2013 and December 31, 2012 are warrants to purchase 700,000 and
736,015 shares, respectively, that are considered to be derivative financial instruments because the warrant agreements contain
&amp;#147;down round&amp;#148; provisions whereby the number of shares covered by the warrants is subject to change in the event of certain
future dilutive stock issuances. The fair value of these derivative instruments at June 30, 2013 and December 31, 2012 was approximately
$909,000 and $5,585,000, respectively, and is included in Derivative Warrant Liability, a current liability, in the Consolidated
Balance Sheet. Changes in fair value of the derivative financial instruments are recognized in the Consolidated Statement of Operations
as a gain or loss on revaluation of derivative warrant liability. The Gain on Revaluation of Derivative Warrant Liability for the
three and six months ended June 30, 2013 was approximately $2,995,000 and $4,676,000, respectively.&amp;#160;&amp;#160;The Gain on Revaluation
of Derivative Warrant Liability for the three and six months ended June 30, 2012 was approximately $370,000 and $601,000, respectively.&lt;/p&gt;

&lt;p style="font: 10pt/normal Times New Roman, Times, Serif; margin: 0"&gt;&amp;#160;&lt;/p&gt;

&lt;p style="font: 10pt/normal Times New Roman, Times, Serif; margin: 0; text-align: justify; text-indent: 9pt"&gt;&amp;#160;The Derivative
Warrant Liability represents the risk exposure pertaining to the warrants and is based on the fair value of the underlying common
stock and the Gain on Revaluation of Derivative Warrant Liability is a result of the change in that fair value.&amp;#160;&amp;#160;For
the three months ended June 30, 2013, no derivative warrants were exercised and none expired. For the six months ended June
30, 2013, no derivative warrants were exercised and 45,164 expired. For the six months ended June 30, 2012, 11,850 derivative warrants
were exercised, which resulted in a reclassification of approximately $42,000 from the Derivative Warrant Liability to Additional
Paid-in Capital.&lt;/p&gt;

&lt;p style="font: 10pt/normal Times New Roman, Times, Serif; margin: 0; text-indent: 0.5in"&gt;&amp;#160;&lt;/p&gt;

&lt;p style="font: 10pt/normal Times New Roman, Times, Serif; margin: 0; text-align: justify; text-indent: 9pt"&gt;&amp;#160;The table below
presents the changes in the Level 3 Derivative Warrant Liability measured for the six months ended June 30, 2013 and 2012:&lt;/p&gt;

&lt;p style="font: 10pt/normal Times New Roman, Times, Serif; margin: 0"&gt;&amp;#160;&lt;/p&gt;

&lt;table cellspacing="0" cellpadding="0" style="width: 100%; font: 10pt Calibri, Helvetica, Sans-Serif; border-collapse: collapse"&gt;
&lt;tr style="vertical-align: bottom"&gt;
    &lt;td style="line-height: 115%"&gt;&amp;#160;&lt;/td&gt;
    &lt;td style="line-height: 115%"&gt;&amp;#160;&lt;/td&gt;
    &lt;td colspan="2" style="border-bottom: black 1pt solid; text-align: center; line-height: 115%"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif; color: black"&gt;&lt;b&gt;2013&lt;/b&gt;&lt;/font&gt;&lt;/td&gt;
    &lt;td style="line-height: 115%"&gt;&amp;#160;&lt;/td&gt;
    &lt;td style="line-height: 115%"&gt;&amp;#160;&lt;/td&gt;
    &lt;td colspan="2" style="border-bottom: black 1pt solid; text-align: center; line-height: 115%"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif; color: black"&gt;&lt;b&gt;2012&lt;/b&gt;&lt;/font&gt;&lt;/td&gt;
    &lt;td style="line-height: 115%"&gt;&amp;#160;&lt;/td&gt;&lt;/tr&gt;
&lt;tr style="vertical-align: bottom; background-color: #CCEEFF"&gt;
    &lt;td style="width: 78%; padding-right: 2.15pt; padding-left: 9pt; text-indent: 9pt; line-height: 115%"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif; color: black"&gt;Derivative Warrant Liability as of January 1&lt;/font&gt;&lt;/td&gt;
    &lt;td style="width: 1%; line-height: 115%"&gt;&amp;#160;&lt;/td&gt;
    &lt;td style="width: 1%; line-height: 115%"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif; color: black"&gt;$&lt;/font&gt;&lt;/td&gt;
    &lt;td style="width: 8%; text-align: right; line-height: 115%"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif; color: black"&gt;5,585,141&lt;/font&gt;&lt;/td&gt;
    &lt;td style="width: 1%; line-height: 115%"&gt;&amp;#160;&lt;/td&gt;
    &lt;td style="width: 1%; line-height: 115%"&gt;&amp;#160;&lt;/td&gt;
    &lt;td style="width: 1%; line-height: 115%"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif; color: black"&gt;$&lt;/font&gt;&lt;/td&gt;
    &lt;td style="width: 8%; text-align: right; line-height: 115%"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif; color: black"&gt;1,035,337&lt;/font&gt;&lt;/td&gt;
    &lt;td style="width: 1%; line-height: 115%"&gt;&amp;#160;&lt;/td&gt;&lt;/tr&gt;
&lt;tr style="vertical-align: bottom; background-color: white"&gt;
    &lt;td style="padding-right: 2.15pt; padding-left: 9pt; text-indent: 27pt; line-height: 115%; font-size: 11pt"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif; color: black"&gt;Total unrealized losses included in net loss &lt;/font&gt;&lt;font style="font: 7pt Times New Roman, Times, Serif; color: black"&gt;(1)&lt;/font&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif; color: black"&gt;&amp;#9;&lt;/font&gt;&lt;/td&gt;
    &lt;td style="line-height: 115%"&gt;&amp;#160;&lt;/td&gt;
    &lt;td style="line-height: 115%"&gt;&amp;#160;&lt;/td&gt;
    &lt;td style="text-align: right; line-height: 115%"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif; color: black"&gt;781,682&lt;/font&gt;&lt;/td&gt;
    &lt;td style="line-height: 115%"&gt;&amp;#160;&lt;/td&gt;
    &lt;td style="line-height: 115%"&gt;&amp;#160;&lt;/td&gt;
    &lt;td style="line-height: 115%"&gt;&amp;#160;&lt;/td&gt;
    &lt;td style="text-align: right; line-height: 115%"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif; color: black"&gt;&amp;#151;&lt;/font&gt;&lt;/td&gt;
    &lt;td style="line-height: 115%"&gt;&amp;#160;&lt;/td&gt;&lt;/tr&gt;
&lt;tr style="vertical-align: bottom; background-color: #CCEEFF"&gt;
    &lt;td style="padding-right: 2.15pt; padding-left: 9pt; text-indent: 27pt; line-height: 115%; font-size: 11pt"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif; color: black"&gt;Total unrealized gains included in net loss &lt;/font&gt;&lt;font style="font: 7pt Times New Roman, Times, Serif; color: black"&gt;(1)&lt;/font&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif; color: black"&gt;&amp;#9;&lt;/font&gt;&lt;/td&gt;
    &lt;td style="line-height: 115%"&gt;&amp;#160;&lt;/td&gt;
    &lt;td style="line-height: 115%"&gt;&amp;#160;&lt;/td&gt;
    &lt;td style="text-align: right; line-height: 115%"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif; color: black"&gt;(5,305,000&lt;/font&gt;&lt;/td&gt;
    &lt;td style="line-height: 115%"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif; color: black"&gt;)&lt;/font&gt;&lt;/td&gt;
    &lt;td style="line-height: 115%"&gt;&amp;#160;&lt;/td&gt;
    &lt;td style="line-height: 115%"&gt;&amp;#160;&lt;/td&gt;
    &lt;td style="text-align: right; line-height: 115%"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif; color: black"&gt;(539,648&lt;/font&gt;&lt;/td&gt;
    &lt;td style="line-height: 115%"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif; color: black"&gt;)&lt;/font&gt;&lt;/td&gt;&lt;/tr&gt;
&lt;tr style="vertical-align: bottom; background-color: white"&gt;
    &lt;td style="padding-right: 2.15pt; padding-left: 9pt; text-indent: 27pt; line-height: 115%; font-size: 11pt"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif; color: black"&gt;Total realized gains included in net loss &lt;/font&gt;&lt;font style="font: 7pt Times New Roman, Times, Serif; color: black"&gt;(1)&lt;/font&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif; color: black"&gt;&amp;#9;&lt;/font&gt;&lt;/td&gt;
    &lt;td style="line-height: 115%"&gt;&amp;#160;&lt;/td&gt;
    &lt;td style="line-height: 115%"&gt;&amp;#160;&lt;/td&gt;
    &lt;td style="text-align: right; line-height: 115%"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif; color: black"&gt;(152,668&lt;/font&gt;&lt;/td&gt;
    &lt;td style="line-height: 115%"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif; color: black"&gt;)&lt;/font&gt;&lt;/td&gt;
    &lt;td style="line-height: 115%"&gt;&amp;#160;&lt;/td&gt;
    &lt;td style="line-height: 115%"&gt;&amp;#160;&lt;/td&gt;
    &lt;td style="text-align: right; line-height: 115%"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif; color: black"&gt;(61,633&lt;/font&gt;&lt;/td&gt;
    &lt;td style="line-height: 115%"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif; color: black"&gt;)&lt;/font&gt;&lt;/td&gt;&lt;/tr&gt;
&lt;tr style="vertical-align: bottom; background-color: #CCEEFF"&gt;
    &lt;td style="padding-right: 2.15pt; padding-left: 0.75in; text-indent: -0.25in; line-height: 115%; padding-bottom: 1pt"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif; color: black"&gt;Reclassification of derivative warrant liability to additional paid-in capital for derivative warrants exercised&lt;/font&gt;&lt;/td&gt;
    &lt;td style="line-height: 115%; padding-bottom: 1pt"&gt;&amp;#160;&lt;/td&gt;
    &lt;td style="line-height: 115%; border-bottom: Black 1pt solid"&gt;&amp;#160;&lt;/td&gt;
    &lt;td style="text-align: right; line-height: 115%; border-bottom: Black 1pt solid"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif; color: black"&gt;&amp;#151;&lt;/font&gt;&lt;/td&gt;
    &lt;td style="line-height: 115%; padding-bottom: 1pt"&gt;&amp;#160;&lt;/td&gt;
    &lt;td style="line-height: 115%; padding-bottom: 1pt"&gt;&amp;#160;&lt;/td&gt;
    &lt;td style="line-height: 115%; border-bottom: Black 1pt solid"&gt;&amp;#160;&lt;/td&gt;
    &lt;td style="text-align: right; line-height: 115%; border-bottom: Black 1pt solid"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif; color: black"&gt;(41,520&lt;/font&gt;&lt;/td&gt;
    &lt;td style="line-height: 115%; padding-bottom: 1pt"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif; color: black"&gt;)&lt;/font&gt;&lt;/td&gt;&lt;/tr&gt;
&lt;tr style="vertical-align: bottom; background-color: white"&gt;
    &lt;td style="padding-right: 2.15pt; padding-left: 9pt; text-indent: 9pt; line-height: 115%"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif; color: black"&gt;Derivative Warrant Liability as of June 30&lt;/font&gt;&lt;/td&gt;
    &lt;td style="line-height: 115%"&gt;&amp;#160;&lt;/td&gt;
    &lt;td style="border-bottom: black 2.25pt double; line-height: 115%"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif; color: black"&gt;$&lt;/font&gt;&lt;/td&gt;
    &lt;td style="border-bottom: black 2.25pt double; text-align: right; line-height: 115%"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif; color: black"&gt;909,155&lt;/font&gt;&lt;/td&gt;
    &lt;td style="line-height: 115%"&gt;&amp;#160;&lt;/td&gt;
    &lt;td style="line-height: 115%"&gt;&amp;#160;&lt;/td&gt;
    &lt;td style="border-bottom: black 2.25pt double; line-height: 115%"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif; color: black"&gt;$&lt;/font&gt;&lt;/td&gt;
    &lt;td style="border-bottom: black 2.25pt double; text-align: right; line-height: 115%"&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif; color: black"&gt;392,536&lt;/font&gt;&lt;/td&gt;
    &lt;td style="line-height: 115%"&gt;&amp;#160;&lt;/td&gt;&lt;/tr&gt;
&lt;tr style="vertical-align: bottom; background-color: white"&gt;
    &lt;td style="line-height: 115%; border-bottom: Black 1pt solid"&gt;&amp;#160;&lt;/td&gt;
    &lt;td style="line-height: 115%"&gt;&amp;#160;&lt;/td&gt;
    &lt;td style="line-height: 115%"&gt;&amp;#160;&lt;/td&gt;
    &lt;td style="text-align: right; line-height: 115%"&gt;&amp;#160;&lt;/td&gt;
    &lt;td style="line-height: 115%"&gt;&amp;#160;&lt;/td&gt;
    &lt;td style="line-height: 115%"&gt;&amp;#160;&lt;/td&gt;
    &lt;td style="line-height: 115%"&gt;&amp;#160;&lt;/td&gt;
    &lt;td style="text-align: right; line-height: 115%"&gt;&amp;#160;&lt;/td&gt;
    &lt;td style="line-height: 115%"&gt;&amp;#160;&lt;/td&gt;&lt;/tr&gt;
&lt;tr style="vertical-align: bottom; background-color: white"&gt;
    &lt;td colspan="8" style="line-height: 115%; font-size: 11pt"&gt;&lt;font style="font: 7pt Times New Roman, Times, Serif; color: black"&gt;(1)&lt;/font&gt;&lt;font style="font: 10pt Times New Roman, Times, Serif; color: black"&gt;&amp;#9;Included in Gain on Revaluation of Derivative Warrant Liability in the Consolidated Statement of Operations.&lt;/font&gt;&lt;/td&gt;
    &lt;td style="line-height: 115%"&gt;&amp;#160;&lt;/td&gt;&lt;/tr&gt;
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