The hemolytic zone that surrounds colonies on blood agar is a characteristic feature that is used in the preliminary identification of group A streptococcus (Streptococcus pyogenes or GAS) in clinical samples. In the 1930s, E. W. Todd distinguished 2 discrete toxins that accounted for the ability of GAS to lyse mammalian erythrocytes [1]. Streptolysin O (SLO) was the designation assigned to the oxygen-dependent labile hemolysin found in cell-free culture fluids during the growth of GAS in liquid cultures. A second cell-associated hemolysin was named streptolysin S (SLS), because its activity was stable to atmospheric oxygen. This latter toxin is responsible for the hemolytic zone around GAS colonies grown under routine aerobic conditions