The α-toxin produced by Clostridium perfringens is one of the best-studied examples of a toxic phospholipase C. In this study, a nontoxic mutant protein from C. perfringens strain NCTC8237 in which Thr74 is substituted by isoleucine (T74I) has been characterized and is compared with the toxic wild-type protein. Thr74 is part of an exposed loop at the proposed membrane-interfacing surface of the toxin. The mutant protein had markedly reduced cytotoxic and myotoxic activities. However, this substitution did not significantly affect the catalytic activity towards water-soluble substrate or the overall three-dimensional structure of the protein. The data support the proposed role of the 70-90 loop in the recognition of membrane phospholipids. These findings also provide key evidence in support of the hypothesis that the hydrolysis of both phosphatidylcholine and sphingomyelin are required for the cytolytic and toxic activity of phospholipases.