Radioactive antibodies that react with tumour-associated antigens to "tag" antigen-positive tumour cell deposits were given to 20 patients with primary or recurrent colorectal cancer. The tumour associated antigen TAG 72-specific monoclonal antibody B72.3 labelled with 125-Iodine was used, and the radioactivity in the tumour was sought during operation with a hand-held gamma detecting probe. Tumour was detected by the probe in 7 of 15 patients with primary cancer, with a mean tumour: normal tissue ratio of 3.9, and in 4 of 5 patients with recurrent disease, with a mean tumour: normal tissue ratio of 2.0. Immunohistochemical analysis of surgical specimens confirmed the results of the intraoperative detection. The incidence of TAG 72-positive tumours (11/20, 55%), detected by immunohistochemistry, was lower than the 80% in the other series, possibly because of sampling errors or because the cases studied were uncomplicated with small primary tumours. Results obtained with the probe were instrumental in modifying the operation in two of the four "positive" patients with recurrences, allowing the removal of tumour masses that would otherwise have been overlooked.