Prostatic acid phosphate (PAP), prostate-specific antigen (PSA), and beta-microseminoprotein (beta-MSP) were regularly localized immunohistochemically to the epithelium of the acini and that of the ducts in the nodules of 24 cases of benign prostatic hyperplasia. The immunohistochemical distribution of these three prostatic-secreted proteins was also examined, with monoclonal antisera against PAP and PSA and with polyclonal antisera against PAP, PSA, and beta-MSP, in a series of 40 cases of prostatic adenocarcinomas graded according to the WHO classification. Highly differentiated (grade I) carcinomas showed a high incidence of PAP-, PSA-, and beta-MSP-immunoreactive cells. As in the normal and hyperplastic prostate parenchyma, highly differentiated (grade I) carcinomas were found to contain an almost equal number of PAP-, PSA-, and beta-MSP-immunoreactive cells. When semiquantitatively assessed, the incidence of PAP-, PSA-, and beta-MSP-immunoreactive cells was found to be lower in the moderately and poorly differentiated (grades II and III) tumors than in the highly differentiated ones; they also showed greater staining variability. Tumor cells immunoreactive with a monoclonal antiserum raised against PAP in carcinomas of grades II and III were less frequent than tumor cells immunoreactive with antisera against PSA, beta-MSP, and a polyclonal antiserum against PAP. The almost identical distribution of PSA and beta-MSP in carcinomas of grades II and III suggests that PSA and beta-MSP are not less sensitive tumor markers than PAP for the monitoring of the course and the treatment of prostatic carcinomas.