The first purpose of this work was to obtain direct evidence that tissue kallikreins cleave arginyl bonds when the leaving group is Arg-Val, and on the contrary, do not split them when it is Arg-Pro; the second aim was to ascertain whether this specificity could be used as a criterion, for characterizing tissue kallikreins. Two tetrapeptides Ac-Phe-Arg-Arg-Val-NH2 and Ac-Phe-Arg-Arg-Pro-NH2 were synthesized by the solid phase method and purified to homogeneity. They were used as substrates for homogeneous preparations of tissue and plasma kallikreins, as well as for some related serine proteases. Products identification and kinetic analyses were made by HPLC.