A human monoclonal anti-idiotypic antibody (105AD7) has been developed which mimics a colorectal-tumour-associated antigen and induces cellular anti-colorectal tumour immune responses in animals. Thirteen patients with advanced colorectal cancer were immunized with 105AD7 and their survival was compared with that of a contemporary group of unimmunized patients with similar disease status. No toxicity related to anti-idiotype immunization was seen. Cellular responses to anti-idiotypic immunization were indicated by lymphocyte proliferation to gp72-positive tumour cells, and production of interleukin-2; anti-tumour antibodies were not detected. Median survival following diagnosis of advanced disease of immunized patients was 12 months, compared with 4 months in unimmunized patients. The improved survival of immunized patients in this study without associated toxicity suggests that 105AD7 immunization may have considerable potential for immunotherapy of colorectal cancer.