Streptolysin S (SLS) is the most potent hemolysin known (14). It is the oxygen-stable hemolysin (104) made by Group A, C, E, G, H, and L streptococci (39). It differs from streptolysin O (SLO) in that 1) the activity once lost cannot be regenerated by reducing agents (48) (15), and 2) it is not antigenic (39). No special substances are required for SLO production (10), but SLS formation depends upon the addition of any of several different substances to the growth media or to harvested cells. The role of these additives on SLS formation or production is uncertain. A brief history of SLS will be presented to illustrate the dependence of SLS upon these additives and to present a nomenclature of SLS.