Mutation of tryptophan-101 in Clostridium difficile toxin A, a 308-kDa glucosyltransferase, resulted in a 50-fold-reduced cytopathic activity in cell culture experiments. The mutant toxin A was characterized and applied to distinguish between glucosyltransferase-dependent and -independent effects with respect to RhoB up-regulation as a cellular stress response.Clostridium difficile toxins A and B (TcdA and TcdB, respectively) are the major pathogenicity factors that are causative for antibiotic-associated pseudomembranous colitis (19). Several reports of the in vivo effects of TcdA in animal models reflect efforts to understand the cellular mechanism leading to clinical symptoms as well as to the release of mediators that are involved in the inflammatory process (2,13,15,18,20). The inherent glucosyltransferase