The antibody response to sheep red blood cells of spleen cells from mice bearing a methylcholanthrene (MCA)-induced tumour was measured with the local hemolysis technique, after in vivo or in vitro immunization. The in vivo data varied according to the stage of tumour growth and were related to the splenomegaly developing during the tumour growth process. During the first 10--15 days after tumour grafting, the total number of plaque-forming cells in the spleen increase in proportion to the spleen growth and thereafter remained relatively constant although the splenomegaly still progressed. Experiments performed in vitro were used to test the antibody response of mixtures of cells from normal mice and syngeneic tumour-bearing animals. In most cases, no suppression of antibody production, due to cells from tumour-bearing hosts, could be detected. The results suggest that those cells, which suppress cell-mediated immune reactions, previously found in the spleen of animals bearing the MCA-induced tumour, do not suppress antibody production.