From 1324 patients with antibiotic-associated diarrhea (AAD) 1643 stool samples were analyzed by a cell test for Clostridium difficile toxin in stool filtrates and cultivation for occurrence of C. difficile strains. In patients with no detectable toxin in their stool strains of C. difficile were isolated in 2.2% whereas when toxin was detectable, the isolation rate varied from 17% to 36%. Furthermore, there was a correlation between toxin titre in stool filtrate and production of cytotoxin in vitro by the corresponding C. difficile strains. Five clostridial strains, not belonging to the species C. difficile, were found to produce typical cytotoxin in vitro. However, five strains identified as C. difficile by biochemical reactions and gas liquid chromatography, did not produce an extracellular cytotoxin. The antibiotic susceptibility patterns of the Clostridium strains were investigated. No correlation was recognized between antibiotic resistance of isolated Clostridium strains and the AAD-inducing antibiotic penicillins and linco/clindamycin. Neither did cases of relapse of diarrheal disease after vancomycin treatment harbour C. difficile strains with increased resistance to vancomycin. It is concluded that the pathogenesis of antibiotic-associated enterocolitis is more complex than a mere intestinal overgrowth of resistant strains of C. difficile.