April 6, 2012

PROLOR Biotech, Inc.

7 Golda Meir Street

Weizmann Science Park

Nes-Ziona, Israel 74140

Re: PROLOR Biotech, Inc. Shelf Registration Statement on Form S-3

Ladies and Gentlemen:

We have acted as legal counsel for PROLOR Biotech, Inc., a Nevada corporation (the “Company”), in connection with the preparation of a Shelf Registration Statement on Form S-3, including the prospectus constituting a part thereof (the “Registration Statement”), being filed by the Company with the Securities and Exchange Commission (the “Commission”) under the Securities Act of 1933, as amended (the “Securities Act”), relating to an unspecified number of shares of common stock, par value $0.00001 per share, of the Company (the “Common Stock”), which may be offered in an aggregate amount not to exceed $75,000,000. The Common Stock may be issued and sold or delivered from time to time as set forth in the Registration Statement, any amendment thereto, the prospectus contained therein (the “Prospectus”) and supplements to the prospectus (the “Prospectus Supplements”) and pursuant to Rule 415 under the Securities Act.

In connection with our representation of the Company, and as a basis for the opinion hereinafter set forth, we have examined originals, or copies certified or otherwise identified to our satisfaction, of the following:

(1) the Registration Statement;

(2) the Company’s Articles of Incorporation, as amended to the date hereof (the “Charter”);

(3) the Company’s Bylaws, as amended to the date hereof (the “Bylaws”); and

(4) such other documents and records and other certificates and instruments and matters of law as we have deemed necessary or appropriate to express the opinion set forth below, subject to the assumptions, limitations and qualifications stated herein.

In rendering the opinions set forth below, we have assumed the genuineness of all signatures, the legal capacity of natural persons, the authenticity of all documents submitted to us as originals, the conformity to original documents of all documents submitted to us as duplicates or certified or conformed copies and the authenticity of the originals of such latter documents.

As to matters of fact material to this opinion, we have relied to the extent we deemed reasonably appropriate upon representations or certificates of officers or directors of the Company, without independently verifying the accuracy of such documents, records and instruments.

We have further assumed that: (i) the Registration Statement and any amendments thereto will be effective under the Securities Act, that no stop orders will have been issued by the Commission with respect to the Registration Statement and that the Registration Statement will comply with all applicable laws at the time the Common Stock is offered or issued as contemplated by the Registration Statement; (ii) an appropriate Prospectus Supplement, free writing prospectus or term sheet relating to the Common Stock offered thereby will have been prepared and filed with the Commission in compliance with the Securities Act and will comply with all applicable laws at the time the Common Stock is offered or issued as contemplated by the Registration Statement; (iii) all Common Stock will be issued and sold in compliance with the applicable provisions of the Securities Act and the securities or blue sky laws of various states and in the manner stated in the Registration Statement and the applicable Prospectus Supplement; (iv) any purchase, underwriting or similar agreement relating to the Common Stock being offered will have been duly authorized, executed and delivered by the Company and the other parties thereto; and (v) in connection with the issuance of any Common Stock, an adequate number of authorized and unissued shares of Common Stock will be available for issuance under the Charter, as then in effect.

PROLOR Biotech, Inc.

April 6, 2012

Page 2

Based upon and subject to the foregoing, and subject to the assumptions, limitations and qualifications stated herein, we are of the opinion that, assuming (i) the taking by the board of directors of the Company of all necessary corporate action to authorize and approve the issuance of the Common Stock and (ii) the due issuance and delivery of the Common Stock upon payment therefor in accordance with the applicable definitive purchase, underwriting or similar agreement, or the Prospectus or a Prospectus Supplement approved by the board of directors of the Company the Common Stock will be validly issued, fully paid and nonassessable.

This opinion letter is limited to the matters stated herein, and no opinions may be implied or inferred beyond the matters expressly stated herein. We assume no obligation to supplement this opinion if any applicable law changes after the date hereof or if we become aware of any fact that might change the opinion expressed herein after the date hereof.

We do not express any opinion herein concerning any law other than the laws of the State of Nevada and the federal laws of the United States.

We hereby consent to the filing of this opinion as an exhibit to the Registration Statement and to the use of the name of our firm therein. In giving this consent, we do not admit that we are within the category of persons whose consent is required by Section 7 of the Securities Act.

Very truly yours,

GREENBERG TRAURIG, LLP

/s/ Greenberg Traurig, LLP