Group B streptococcal cells, either viable or heat-killed, contain a substance that induced fever in rabbits with maximal responses occurring four hours after intravenous injection. In contrast, supernatant fluids failed to induce significant fever. Group B streptococcal cells also enhanced host susceptibility to lethal shock by endotoxin as much as 40,000-fold. A graph of log streptococcal cell dose used for pretreatment versus log LD50 endotoxin gave a straight line with a slope of approximately -1. Rabbits that received both streptococcal cells and endotoxin showed initial fever followed by hypothermia, labored breathing, watery diarrhea, evidence of vascular collapse, and finally death. Animals that received streptococcal cells or endotoxin alone showed only fevers and mild diarrhea. A possible theory for the cause of death in the neonate infected with group B streptococci is presented.