Cancer chemotherapeutic agents modify the human immune system in diverse ways including both immunosuppression and immunostimulation. We evaluated the effects of mitomycin-C on rat Kupffer cell phagocytosis, C3b receptor binding, and lysosomal enzyme activity. Kupffer cell cultures were greater than 95% pure. Phagocytosis of IgG-coated sheep red blood cells was demonstrated by 84% of control cells but by only 25% of cells isolated two weeks following mitomycin-C administration (p less than 0.0005). This depression in phagocytic ability had returned to control levels by four weeks posttreatment. Similarly, C3b receptor binding of IgM and complement coated sheep red blood cells was observed in 88% of control Kupffer cells, but declined to 47% at two weeks after drug administration (p less than 0.005) and returned to normal after four weeks. Lysosomal enzyme activity was not impaired by mitomycin-C. Histologically severe ulceration of the colon of treated animals was seen one and two weeks after drug administration, but healed by four weeks post-mitomycin-C treatment. Depression of macrophage function as a consequence of cancer chemotherapy may have important clinical consequences in host defense against bacteria and tumor metastases.