Hybridoma-derived monoclonal anti-colorectal carcinoma antibodies suppressed the growth of colorectal carcinoma in nude mice as evidenced by a lower incidence of tumors, a longer latency period, and a smaller volume of tumors in antibody-treated than in control animals. The growth-inhibiting properties of monoclonal anti-colorectal antibodies seem to be specific for colorectal carcinoma cells. This is indicated by the lack of effect of the antibodies on the growth of melanomas or bronchogenic carcinomas and by the binding of the antibodies in vivo to colorectal carcinoma cells but not to lung or kidney cells from tumor-bearing animals or to other tumor cells implanted in other animals. Inhibition of tumor growth was most probably mediated by antibody-dependent cell-mediated cytotoxicity. The results of these studies could provide an approach to the study of immunotherapeutic possibilities for anti-colorectal carcinoma antibodies in humans.