Immunoscintigraphy with radiolabelled anti-carcinoembryonic antigen monoclonal antibody was performed on 21 patients with active small-cell lung cancer. Patients received either In-111-labelled Mab F6 F(ab')2 fragments or Tc-99m-labelled BW 431/26 intact antibody. Tumour was imaged in 13 patients (62%). Of 38 known sites of disease, 18 sites were positively detected. Serum CEA levels were known in 19 patients and were abnormally elevated in three (tumour being detected in all three patients). Eight of 15 patients with normal serum CEA had positive imaging. Using the In-111-labelled antibody seven out of ten patients (nine out of 18 sites) gave positive scans; with the Tc-99m-labelled antibody these were obtained in nine out of 18 patients (nine out of 20 sites).