• Pertussis vaccine contains lipopolysaccharide (endotoxin). Polymyxin B sulfate neutralizes endotoxin activity in vivo and in vitro from Enterobacteriaceae and Salmonella-derived endotoxin. In vitro, polymyxin B eliminates the endotoxin reaction of pertussis vaccine in the Limulus lysate test. In this study, platelet and WBC counts and antibody response were compared in rabbits given either pertussis vaccine alone or pertussis vaccine and polymyxin B intravenously. Pertussis vaccine—induced leukopenia and thrombocytopenia were eliminated in the polymyxin B group. The antibody titers in the animals receiving pertussis vaccine and polymyxin B were somewhat lower and rose more slowly. Since the toxicity of pertussis vaccine is related in part to endotoxin, we suggest that a clinical study using a combination of the vaccine with an endotoxin-neutralizing agent be done to assess both amelioration of side effects caused by the vaccine and any effect on immunogenesis.