Granulomatous prostatitis and poorly differentiated prostate carcinoma. Their distinction with the use of immunohistochemical methods. Granulomatous prostatitis and poorly differentiated prostate carcinoma can mimic each other both clinically and histologically. To develop criteria useful in resolving problem cases, the authors compared the reactivities of these conditions (nine cases of granulomatous prostatitis and six cases of poorly differentiated carcinoma) with a panel of antibodies to cytokeratin (AE1/3), prostate-specific antigen (PSA), prostatic acid phosphatase (PAP), lysozyme, antimacrophage M, and leukocyte common antigen (LCA). In granulomatous prostatitis, histiocytes were not immunoreactive for PAP, PSA, or cytokeratin; however, histiocytes reacted to lysozyme in nine of nine cases, antimacrophage M in seven of nine cases, and LCA in one of nine cases. Tumor cells from all six carcinoma cases reacted with PAP, PSA, and cytokeratin; all failed to react with lysozyme, LCA, and antimacrophage M. The authors conclude that granulomatous prostatitis and poorly differentiated carcinoma can be reliably distinguished with immunohistochemical methods. 