To detect metastases in the bone marrow of patients with small cell lung cancer, immunofluorescence with a monoclonal antibody detecting a membrane antigen (MOC-1) associated with small cell lung cancer was performed on 53 bone marrow aspirates from 30 patients. In 19 (63%) patients MOC-1 reactive cells were detected. Simultaneous histopathological examination of the bone marrow biopsy specimens detected tumour cells in only six (20%). The method is more sensitive than conventional histochemical staining of bone marrow aspirate and may eventually be able to show additional subgroups, such as patients with limited disease who might benefit from adjuvant radiotherapy or surgery.