Tissues of prostatic origin representing variable phenotypes were tested for reactivity to the prostate cancer specific mouse monoclonal antibody PD-41. Avidin biotin immunoperoxidase was applied on formalin-fixed, paraffin-embedded tissue sections of 15 benign prostatic hyperplasia (BPH), 23 prostatic intraepithelial neoplasia (PIN), 14 untreated primary adenocarcinoma, 35 diethylstilbestrol (DES) treated tumors, 50 lymph node and 11 bone metastases. Specimens were stratified according to the percentage of tumor cells expressing PD-41 antigen and degree of staining intensity, and correlated with PIN grade, Gleason score, flow cytometry (FCM) measured DNA ploidy, and reactivity to other antibodies. In PIN, 4 specimens (17.4%) showed reactivity in a significant number of cells while a few cells were reactive in most cases. PD-41 was significantly reactive (>5% of tumor cells) in 88% of nodal metastases and in 73% of bone metastases in contrast to 49% reactivity in primary tumors (p=0.0003). There was a tendency of increased antigen expression in hormonally treated primary tumors. In addition, involutional and metaplastic changes in hormonally treated cases were reactive in many instances. Semi-quantitative evaluation of PD-41 reactivity showed a statistically significant correlation with Gleason score in primary tumors (p=0.007) and in lymph node metastases (p=0.009). Moreover, the PD-41 antibody reacted in metastatic lesions that failed to express both prostatic acid phosphatase and prostate specific antigen. These data suggest that monoclonal antibody PD-41 merits further investigation to evaluate its potential diagnostic, prognostic and therapeutic role in prostate cancer.