Tissues from patients with benign prostatic hyperplasia were used for the cytochemical demonstration in light and electron microscopy of a secreted, nonlysosomal prostatic acid phosphatase (PAP) with phosphorycholine, substrate specific for PAP. The specificity of phosphorylcholine for PAP is attributable to the pentavalent nitrogen in phosphorylcholine, a feature that renders it resistant to hydrolysis by all other acid phosphatases. PAP activity was found in the Golgi cisternae and its associated vacuoles and in secretory vacuoles localized in the nuclear, Golgi, and apical areas of the prostatic epithelial cell. These results confirm the existence of two types of acid phosphatase in prostatic tissue. One is lysosomal and is prevalent in many tissues and the other, PAP, is the major enzymatic product secreted by the prostate. The specificity of PAP for phosphorylcholine, one of the natural substrates for this enzyme, validates the use of this method for the histochemical characterization of PAP and indicates the prostatic origin of cells showing PAP activity.