Basal cell hyperplasia (BCH) is an uncommon proliferative lesion of the prostate gland. We studied ten cases of BCH, one case of an unusual adenoid basal cell tumor (ABT), and one case of a prostatic adenoid cystic carcinoma (ACC), using a panel of antibodies to define the histogenesis of these lesions. Monoclonal antibodies (MoAb) directed against a cytokeratin, which selectively stains basal cells (34 beta E12), and against muscle-specific actin, which stains myoepithelial cells (HHF35), were used. In addition, antibodies directed against prostatic acid phosphatase (PAP), prostate-specific antigen (PSA), S-100 protein, and vimentin were used. In the normal prostate, epithelial cells reacted positively with 34 beta E12, PAP, and PSA, and negatively with the actin, S-100 protein, and vimentin antibodies. In BCH, positive staining was seen for 34 beta E12, PSA, and PAP, with no reactivity for actin, S-100 protein, and vimentin. In ABT and ACC, positive reactivity was demonstrated for all antibodies except actin and vimentin. These findings indicate that the basaloid cells of BCH, ABT, and ACC are derived from basal cells of the normal prostate gland and suggest a continuum among the three lesions. The presence of S-100 protein in ABT and ACC may be related to the lack of this antigen's specificity for myoepithelial cells. The absence of reactivity with the HHF35 MoAb supports our belief that the S-100 positivity does not necessarily indicate myoepithelial cell differentiation.