A panel of five monoclonal antibodies to surface antigens of small-cell pulmonary carcinoma was used for immunodiagnosis of metastases to the bone marrow in vitro in patients with small-cell carcinoma of the lungs. Standard cytomorphological diagnosis revealed metastases to the bone marrow in 6 out of 58 patients (10.3%). Using monoclonal antibodies, positive cells were detected in 12 (20.6%) patients, the tumor nature of the positive cells being undoubted in 7 preparations (12.6%). Five out of 6 cytologically positive cases were confirmed.