Of the total of 306 amino acids in the sequence of sphingomyelinase (SMPLC) from Bacillus cereus, almost half (150) are expected to be involved in the formation of loop or turn structure, while 65 and 73 residues may participate in the formation of alpha-helix and beta-structure, respectively. The helix content of SMPLC was calculated to be 0-5%, based on the CD spectra. The addition of divalent metal ions such as Mg2+ or both Ca2+ and Mg2+ had no effect on the CD spectra of SMPLC, although the addition of these metal ions caused the breakdown of membranous SM and specific adsorption of SMPLC onto erythrocyte membranes. A hydropathy study showed that SMPLC has hydrophobic regions at the N-terminal domain which must be responsible for the binding of the enzyme to the membranes. The partial homologies between the amino acid sequences of SMPLC and Clostridium perfringens alpha-toxin (phospholipase C) are discussed.