HISTORICAL PERSPECTIVE Kayser in 1903 reported that some E. (oli cultures lysed erythrocytes (RBC) (55). He noted that the culture supernatants retained hemolytic activity after being filtered through a Chamberland filter. Dudgeon and Pulvertaft in 1927 also reported hemolytic activity in E. coli cultures (19) but could not demonstrate hemolysin in culture filtrates. They assumed that all activity was associated with the E. coli cell. Several other reports confirmed the existence of cell-associated hemolytic activity (2, 19, 20, 51, 81, 86). Kayser's report of a filterable hemolysin was confirmed in 1960 when Lovell and Rees (64) obtained a bacteria-free hemolysin preparation by filtration of cultures grown in alkaline meat infusion broth. Smith in 1963 was the first to clearly differentiate cellbound and cell-free hemolysin in cultures of E. (oli grown in alkaline meat extract broth (88). He showed that under the same growth conditions some hemolytic strains of E. coli produce cell-free and cell-bound hemolysin simultaneously. Cell-bound hemolysin was not neutralized by antiserum prepared against the cell-free hemolysin, indicating that the two hemolysins might be different. Smith designated the cell-bound hemolytic factor as P-hemolysin and the cell-free factor as oa-hemolysin (AH). This was perhaps an unfortunate designation and has caused some confusion since both the ot-and ,-hemolysins cause 3-hemolysis (clear zone of lysis) around colonies on blood agar plates. Walton and Smith (113) found a third hemolysin (yhemolysin) produced by mutants resistant to nalidixic acid. Hemolysin production by these mutants is enhanced by growth in nalidixic acid. Unlike the cxand 3-hemolysins, the 326