A panel of monoclonal antibodies against neural and epithelial associated antigens was used to examine bone marrow from patients in clinical remission from small cell lung cancer (SCLC). A standard peroxidase-antiperoxidase technique and Ficoll-Hypaque enrichment were used to detect SCLC-like cells at the 1-2% level of contamination in 8 of 12 patients who were disease free by conventional criteria, including routine marrow cytology and histology and endobronchoscopic biopsy or cytology. Six of these patients ultimately relapsed, with metastatic sites found between 2 and 6 months after restaging. Furthermore, 6 patients had undergone chemointensification including autologous marrow rescue with radical irradiation to the primary lung tumor. Four of these 6 subsequently relapsed, also with metastatic sites. Of the 4 patients without bone marrow metastases at restaging using this technique, 2 relapsed, with cells found at the primary site, and 2 remained in complete remission. Serum free cell culture was attempted in 9 of 12 cases and SCLC-like cell colonies grew, in suspension, in 4. The SCLC-like nature of these cells has been confirmed by electron microscopy in 1 case and by repeat immunocytochemistry for small cell associated antigens in 3 cases. Bone marrow positivity using these techniques appears to predict a high risk of metastatic relapse regardless of further therapy.