Specimens from 30 cases of benign prostatic hyperplasia and 75 cases of prostatic carcinoma obtained during suprapubic prostatectomy, transurethal resection of the prostate and radical prostatectomy, were stained immunohistochemically for S-100 protein, prostatic acid phosphatase (PAP), prostatic specific antigen (PSA), neuron specific enolase (NSE) and polyclonal keratin. S-100 protein was positive in 9.3% of prostatic carcinomas and negative in all cases of prostatic hyperplasia. PAP and PSA were positive in all cases, while NSE was positive in 16% of the carcinoma cases. Polyclonal keratin was positive in both cell layers of the double layered hyperplastic prostatic epithelium with a more intense staining pattern in the outer cell layer. The authors believe that the S-100 protein immunoreactivity observed in some prostatic carcinomas, reflecting the change in the functional status of the neoplastic cells, might be of prognostic significance. They also emphasize the non-myoepithelial nature of the outer cell layer of the double layered prostatic epithelium.