N alpha substituted 4-cyanophenylalanines were prepared by reaction of the acid chloride, the activated ester and the mixed anhydride of Tos-Pro-OH, 2-naphthylsulfonyl-L-proline and Z-Pro-OH, respectively, with cyanophenylalanine. These acids were transferred into the amides via the 4-nitrophenyl esters or the mixed anhydrides. The cyano compounds were converted via the thioamides and the thioimidic esters into the amidines by common way. The exchange of the glycine residue by prolin in compounds former described caused a decreased antithrombin effect.