In a case of collagenous colitis, cholestyramine treatment resulted in symptomatic and histological normalization. After discontinuation of cholestyramine, collagenous colitis relapsed. At this time fecal cytotoxic activity was demonstrated in McCoy cell lines. Symptoms, histologic changes, and cytotoxicity disappeared when cholestyramine treatment was reinstituted. We hypothesize that a bacterial toxin is responsible for the development of collagenous colitis.