We investigated the proliferative responses of spleen cells (SC) to polyclonal mitogens lipopolysaccharide (LPS) and pokeweed mitogen (PWM), immune responses to sheep red cells (SRC) in mice undergoing hyperthermia. There were increased proliferative responses of lymphocytes to PWM if we used mice having rectal temperature 42 degrees C. Thermal shock in mice was accompanied by suppression of immune response. If we used mice suffering from hyperthermia (43-44 degrees C) for 20 minutes; there were decreased proliferative responses of lymphocytes to PWM or LPS for 10-30 days. We observed low immune response to sheep red cells in mice for 5-20 days. The changes of immune response were not revealed on the 40th day after induction of hyperthermia in mice.