We used a mixture of antisera to prostatic specific acid phosphatase, prostatic specific antigen, epithelial membrane antigen and cytokeratin to examine multiple marrow aspirates from patients with local (15) and metastatic prostatic carcinoma (15), and benign prostatic hypertrophy (10). We found moderate to large numbers of tumor cells in the bone marrow of 11 of 15 (73 per cent) patients with known metastatic disease and small numbers of abnormal cells in 2 of 15 (13 per cent) patients with apparently local disease. No tumor cells were found in patients with benign prostatic hypertrophy, and only 2 patients with metastatic disease had tumor cells in the bone marrow when conventional hematomorphological preparations were examined. These findings suggest that immunocytochemistry can increase the detection rate of metastatic prostatic carcinoma cells. Further followup of larger numbers of patients with local carcinoma will reveal whether the presence of micrometastases denotes a poor prognosis.