This investigation is a part of a phase three multicentre immunoscintigraphy study of a technetium-99m labelled monoclonal antibody (Mab), BW 431/26, directed against carcino-embryonic antigen (CEA). Ten patients with recently colonoscopically verified primary colorectal cancer were evaluated. In nine of ten patients colorectal cancers were detected by wholebody immunoscintigraphy and by single photon emission computerized tomography (SPECT). The ten patients were operated upon, and in nine cases immunohistochemical investigations were performed. Immunohistochemical investigation of nine tumour specimens show that CEA was present in much higher concentrations in the tumour than in normal intestinal epithelium. Active accumulation of the labelled Mab in tumours was demonstrated by scintimetry. There is a demand for new diagnostic techniques for early diagnosis of colo-rectal cancer. Immunoscintigraphy must however compete with other diagnostic possibilities. The development of immunoscintigraphic technique is proceeding very rapidly, and there is probably a future for the use of immunological methods in diagnosis and therapy of cancer.