223 sera of patients having a colonic cancer or other diseases were examined for the presence of antibodies agglutinating tanned sheep erythrocytes coated with saline extracts of human colonic cancers. Some patients' sera were also investigated for precipitating antibodies. Many positive results were found, mainly but not only, in colonic cancer patients' sera. By absorption studies the antibodies found in these sera have been shown to be directed against an antigen present not only in the tumours but also in normal and foetal tissues as well as in the microsomal fraction of the intestine. The chemical nature of the antigen is still under investigation.