Using the intact monoclonal IgG1 anti-CEA antibody BW 431/26 (Behringwerke Marburg, FRG) labelled with 99mTc by a new labelling procedure (Schwarz method), 72 patients suspected of tumour recurrences (colorectal cancer, n = 59) and with rising serum carcinoembryonic antigen (CEA) levels were studied. Sixty-nine of 72 studies had a positive result by immunoscintigraphy of which 52 have since been proven histologically/clinically. In six patients only immunoscintigraphy revealed a tumour recurrence while all other diagnostic modalities were negative. Of 52 studies 46 were true positive (diagnostic sensitivity 88%). In this series there was one false positive result. About one-third of all patients injected with the antibody developed a human anti-mouse response (HAMA), but no severe side effects were seen even after four applications. For the follow-up of colorectal and other CEA positive adenocarcinomas this 99mTc-labelled monoclonal antibody seems to be very promising in patients with rising serum CEA, especially if conventional diagnostic imaging procedures remain negative.