While using the natural killer (NK) cell-associated HNK-1 antibody in a panel of hematopoietic cell markers, the authors found that metastatic tumor cells in the bone marrow of a patient with disseminated prostatic carcinoma stained strongly with this antibody using an indirect immunofluorescence technique. More than 50% of cells from the patient's prostate also reacted with HNK-1. Subsequent study of frozen sections of prostate tissue from patients with benign prostatic hypertrophy (BPH) showed that HNK-1 reacted with prostatic epithelium and the contents of the glandular lumina. Two other markers associated with NK cells (OKM1 and OKT3) were not detected on nonneoplastic or neoplastic prostatic epithelial cells using a two-color immunofluorescence technique. Recent reports have shown that the HNK-1 antibody also detects an antigen on cells of neuroectodermal origin. The authors have concluded that HNK-1 also reacts with prognostic epithelium in patients with BPH and may be useful as another marker for metastatic carcinoma of the prostate.