The monoclonal antitumour antibody 791T/36, previously shown to localize in colorectal cancer when radiolabelled with 131I, has been successfully labelled with 111In. A preliminary evaluation of its imaging potential has been made in fourteen patients with primary and secondary colorectal cancer. Positive localization was achieved in tumour sites of all but one of the patients. The sites of uptake of the 111In-labelled antibody were more clearly defined than when using the same antibody labelled with 131I. The mean tumour to normal tissue uptake ratio per gram of tissue from four resected specimens was 2.8:1. Labelling of the antibody did not affect the tumour uptake of the antibody but facilitated interpretation directly from images without the need for background subtraction.