Summary Certain esters of arginine, but not those of a number of other amino acids, are hydrolyzed rapidly by some lobes of the prostate gland of a number of species. Marked differences in the activity of this arginine ester hydrolyzing enzyme in different lobes of the prostate gland of various rodents have been observed. Dog prostatic fluid is extremely rich in this enzyme, whereas the human prostate hydrolyzes arginine esters rather slowly. The activity of the enzyme in rodent prostatic tissue is depressed after castration. Some properties of this enzyme, and its relationship to other proteolytic enzymes in prostatic secretion as well as to the process of semen coagulation are discussed.