Guinea pigs treated with lincomycin developed colitis, acute cholecystitis and abnormalities in red blood cell morphology. The present study was designed to study the production of clostridial toxins after lincomycin treatment. Lincomycin produced abnormalities in conventional but not in germ-free guinea pigs. Clostridium difficile was cultured from cecal contents of conventional guinea pigs treated with lincomycin. Cecal filtrate from sick guinea pigs was subjected to Sepharose 4B-CL and Sephadex G-200 column chromatography, yielding a partially purified toxin. Both cecal filtrate and partially purified toxin samples contained a heat labile substance which was cytotoxic to human lung fibroblast Wl-38 cells, and which was neutralized by Clostridium sordelli antitoxin. Toxin samples given orally or intraperitoneally killed normal guinea pigs. Finally, toxin samples induced red cell membrane changes in vitro as well as producing features of acute inflammation in healthy explants of guinea pig cecum and gallbladder in organ culture. Lincomycin treated guinea pigs produced Clostridium difficile toxin(s) which in turn caused diffuse cell membrane injury.