As previously reported (1, 2) phosphorylcholine (PC) is a specific substrate for prostatatic acid phosphatase (PAP) as opposed to other acid phosphatases, e.g., lysosomal acid phosphatase. The specificity of PC for PAP is due to the pentavalent nitrogen in PC, a feature that renders PC resistant to hydrolysis by all other acid phosphatases. Detailed comparative cytochemical results in rat tissues are in press. This report deals with ultracytochemical results applying the method to normal and pathological human prostate gland.Fresh human prostate was obtained from 7 patients having transurethral resections or radical prostatectomies. The tissue was fixed in 3% glutaraldehyde- 0.1 M cacodylate buffer (pH 7.4) for 15 min, sectioned at 50 μm on a Sorvall TC-2 tissue sectioner, refixed for a total of 2 hr, and rinsed overnight in 0.1 M cacodylate buffer (pH 7.4)-7.5% sucrose.