X0306 4 2013-04-10 0 0001443669 Proto Labs Inc PRLB 0001183159 KRANTZ DONALD G 5540 PIONEER CREEK DRIVE MAPLE PLAIN MN 55359 0 1 0 0 Chief Operating Officer Common Stock 2013-04-10 4 M 0 42000 7.86 A 43250 D Common Stock 2013-04-10 4 S 0 29650 50.2814 D 13600 D Common Stock 2013-04-10 4 S 0 12350 51.2178 D 1250 D Employee Stock Option (right to buy) 7.86 2013-04-10 4 M 0 42000 0.00 D 2020-12-21 Common Stock 42000 63000 D Transaction effected pursuant to a Rule 10b5-1 trading plan adopted by the reporting person on February 15, 2013. The transactions reported on this Form 4 conclude the transactions contemplated by the Rule 10b5-1 trading plan. Reflects the weighted average price of 29,650 shares of common stock of Proto Labs, Inc. sold by the reporting person in multiple transactions on April 10, 2013 with sale prices ranging from $50.00 to $50.99 per share. The reporting person undertakes to provide upon request by the U.S. Securities and Exchange Commission staff, the issuer, or a security holder of the issuer, full information regarding the number of shares sold at each separate price. Reflects the weighted average price of 12,350 shares of common stock of Proto Labs, Inc. sold by the reporting person in multiple transactions on April 10, 2013 with sale prices ranging from $51.00 to $51.60 per share. The reporting person undertakes to provide upon request by the U.S. Securities and Exchange Commission staff, the issuer, or a security holder of the issuer, full information regarding the number of shares sold at each separate price. The option vests as to 20% of the shares in five annual installments beginning on December 21, 2011. /s/ Julie M. Regnier, Attorney-in-Fact for Donald G. Krantz 2013-04-12