There are potent kinin degrading activities in seminal plasma and testis of various mammals. The activities in boar and human seminal plasma, and testis extracts from boar, rat, guinea pig and rabbit were eluted out at a similar position as a single peak in column chromatography with Sephadex G-200 or DEAE Sephadex A-50. These enzymes degraded synthetic bradykinin and yielded angiotensin II from angiotensin I by cleaving the second peptide bond from the carboxytermini of the substrate, and it was concluded that these enzymes were dipeptidyl carboxypeptidases like kininase II or angiotensin I converting enzyme. The enzymes in male genital organs of these mammals were found to be identical with further investigation on the enzymic properties of them. The enzyme in rat testis increased significantly in accordance with sexual maturation, and the increase was suppressed by injection of danazol or diethylstilbesterol to rats. Furthermore, the dipeptidyl carboxypeptidase content in human seminal plasma was positively correlated to the semen qualities, i.e. sperm density and motility. From these results it is supposed that dipeptidyl carboxypeptidase in male genital organ and its secretion seem to be related to the male reproductive functions.