Abstract Horse urinary kallikrein when incubated with horse plasma formed kallidin (lysylbradykinin) from the kininogens in the plasma. Horse plasma, like human plasma, was found to contain an aminopeptidase capable of converting kallidin to bradykinin. No evidence, however, could be found that the plasma contained an aminopeptidase capable of converting Met-Lys-bradykinin to kallidin, thus eliminating the possibility that the kallikrein had released Met-Lys-bradykinin which was converted to kallidin during the 1–5 min incubations. The method used for identification of the kinins is rapid, gives a good recovery and requires small amounts of plasma and enzyme.