Tumour-associated HMFG2 monoclonal antibody (MAb) was labelled with indium-111 and administered intravesically to 20 patients with known or suspected superficial bladder carcinoma. The antibody solution was kept in the bladder for 1 hr and was then washed out. Cystoscopy was performed at 2 and 24 hr after instillation. Radioactivity of tumour and normal tissue obtained from the bladder during cystoscopy and cells recovered from urine after the instillation were counted in a gamma-counter. Conventional histology, immunocytochemistry and autoradiography were also performed. Mean uptake at 2 and at 24 hr was higher in tumours than in normal samples. Autoradiography showed selective accumulation of radioactivity in cells which expressed the antigen detected by the HMFG2 MAb. There was no correlation of tumour uptake with the grade of tumour. No radioactivity was found in the blood of patients after the instillation. Based on dosimetric calculations, however, the radiation dose that can be delivered to the tumours is not sufficient to be cytotoxic, possibly due to inadequate penetration and retention by tumour tissue. Nevertheless, the significant difference between antibody uptake by the tumours and that by normal urothelium, observed in our study, allow for the possibility of using this approach therapeutically.