Sphingomyelinase of Bacillus cereus was specifically adsorbed onto sphingomyelin liposome in the presence of Ca2+ or with the coexistence of Ca2+ and Mg2+, but not onto the liposome of phosphatidylcholine. In the presence of Ca2+, the enzyme adsorption onto bovine erythrocytes and liposome increased with an increase in the amount of sphingomyelin. These results support that the major binding site for sphingomyelinase in the erythrocytes is sphingomyelin. The temperature-dependence of enzyme adsorption was not influenced by a change in ATP content of bovine erythrocytes. After treatment of red cells with neuraminidase or pronase, enzyme adsorption at 0 degrees C lower than that at 37 degrees C was observed. In unsealed or right-side-out ghosts, the difference between the enzyme adsorption at 37 degrees C and that at 0 degrees C became less pronounced than in the erythrocytes. Furthermore, the extent of enzyme adsorption onto sphingomyelin liposome at 0 degrees C was almost equal to that at 37 degrees C. The enzyme adsorption onto the erythrocyte membrane and liposome was always enhanced in the presence of Ca2+; in the presence of Mg2+ alone, adsorption was observed only for erythrocyte ghosts and the adsorbed enzyme was released from the membrane after extensive degradation of sphingomyelin by sphingomyelinase. With the coexistence of Ca2+ and Mg2+, the enzymatic hydrolysis of sphingomyelin proceeded rapidly for the attack against liposome and all the membranes tested, whereas in the presence of Mg2+ alone, hydrolysis was observed only for the action of liposome and ghosts. No appreciable hydrolysis of sphingomyelin was observed in the presence of Ca2+ nor in the absence of divalent metal ions.