The authors investigated the proteolytic activity of the normal prostate, the prostate with benign hypertrophy and with cancer. Pepsinogen obtained from the normal prostate was isolated on DEAHP pearl cellulose and purified by means of affinite chromatography. Separation on affinite chromatography and by electrophoresis on agar confirmed that the pepsinogen is identical with gastric progastricsin. The proteolytic activity was examined by a modified method of Anson and Mirsky, and with the aid of synthetic substrates it was assessed quantitatively in all three examined groups. The enzyme activity of the carcinomatous tissue practically did not differ from the activity of the normal prostate. In beniming hypertrophy of the prostate the proteolytic activity assessed by the breakdown of haemoglobin is markedly reduced and when tested on the breakdown of synthetic substrates [For-Tyr-Tyr- and Ac-Phe- Tyr (I2)] it is practically zero.