The NH2-terminal domain of the alpha-toxin of Clostridium perfringens is highly homologous to the complete phospholipase C from Bacillus cereus (PC-PLC), for which a high-resolution crystal structure is available. This structural information was used as the basis of a site-directed mutagenesis strategy in which critical amino acid residues of alpha-toxin involved in zinc binding, interaction with substrate, or catalysis were replaced. Biochemical studies with the corresponding toxin variants indicate that there is probably a single active site endowed with lecithinase, sphingomyelinase, and hemolytic activities. By using a highly purified variant in which the catalytic aspartate residue at position 56 was replaced by asparagine, it was shown that phospholipase activity was essential for lethality in vivo and for mediating platelet aggregation in vitro.