Following oral administration of lincomycin, over 50% of two groups of 12 rabbits each died between 4 and 56 days with distended, non-hemorrhagic, fluid-filled ceca. Bacteria-free cecal filtrates from the rabbits that died were lethal for mice, cytopathic in Y-1 tissue culture monolayers, and caused increased vascular permeability in rabbit skin. Although the cecal filtrates of both groups had similar biological activity, the filtrate activity of one group was neutralized by Clostridium perfringens iota antitoxin, and the infiltrate activity of the other group was neutralized by Clostridium difficile antitoxin. Toxigenic Clostridium difficile also were isolated from the ceca of this group. A broth filtrate of a Clostridium difficile culture was lethal for rabbits when injected intravenously and intraperitoneally. The data indicated Clostridium difficile toxin or clostridium perfringens iota toxin may be associated with lincomycin-associated colitis in rabbits.