Suppressor cells in tumor bearing mice and rats.

Suppressor macrophages in spleens of tumor bearing mice and rats were found to inhibit "in vitro" reactivity of lymphocytes to T and B cell mitogens and to allogeneic cells in the mixed lymphocyte tumor cell interaction and the "in vitro" generation of secondary cytotoxic effector cells. We propose that similar suppressor cells may play an important role in the general immunosuppression of tumor bearing hosts and in the inability of tumor bearing hosts to effectively reject the tumor.