Adoptive immunotherapy with lymphokine-activated killer (LAK) cells has shown some promise in the treatment of certain cancers that are unresponsive to conventional treatment approaches. However, colon adenocarcinomas tend to respond poorly to LAK therapy, possibly as a result of tumor-induced immunosuppression. Recently, in vivo administration of anti-CD3 antibody has been shown to induce mouse T lymphocytes to mediate major-histocompatibility-complex(MHC)-unrestricted tumoricidal activity which is distinct from natural-killer-cell-derived LAK activity. It has therefore been suggested that anti-CD3 therapy may find application in tumor immunotherapy in humans. However, the effectiveness of anti-CD3-activated killer cell induction within the environment found in the vicinity of colon adenocarcinoma cells has not been evaluated. The present report demonstrates that colon cancer cells of human (HT-29) and mouse (MCA-38) origin markedly inhibit the generation of activated killer cells in murine spleen cell cultures. DNA synthesis and interleukin-2 production by spleen cells following stimulation with anti-CD3 antibody are also profoundly depressed in the presence of MCA-38 and HT-29 adenocarcinoma cells. MCA-38- and HT-29-mediated inhibition of activated killer cell development is exerted through the production of a tumor-associated soluble factor that is distinct from transforming growth factor beta or prostaglandins. Local immunosuppression associated with sites of tumor growth may therefore represent a major obstacle to successful anti-CD3 immunotherapy of certain colon adenocarcinomas.