ASSAY O F STAPHYLOCOCCAL 8-HAEMOLYSIN WITH

they are differentiated by their lytic activity against erythrocytes of different animal species. For the assay of a-haemolysin (Arbuthnott, 1970) and y-haemolysin (Mollby and Wadstrom, 1971; Wiseman, 1975) rabbit erythrocytes are used because they are more sensitive than all others that have been tested. The,8-haemolysin, a sphingomyelinase C (Doery et al., 1965) is assayed with sheep or bovine erythrocytes; they are particularly sensitive because of their high content of sphingomyelin (Nelson, 1967; Wiseman and Caird, 1967). Delta-haemolysin has a much lower specific activity than the other haemo-lysins, and in low dilutions lyses a wide range of erythrocytes; it is usually assayed with human or horse erythrocytes and these have indicated as many as 300 haemolytic units per mg (HU per mg) in purified preparations (Heatley, 1971; Kreger et al., 1971). Heatley (1971) found that,8-haemolysin was capable of interfering with the assay of 8-haemolysin when human erythrocytes were used. Lysis of bacterial protoplasts (Bernheimer et al., 1972) provides a more sensitive assay (610 units per mg) but is less convenient when a large numbe