Cysteine and other sulfhydryl compounds markedly enhance the formation of streptolysin S by 4 strains of group A streptococci. In the presence of cysteine (0.001 M) 10,000-30,000 hemolytic units per ml were obtained from strain S 84 type 3 in comparison with 1,000-3,000 units in the absence of cysteine. The role of cysteine in formation of hemolysin is not clear but it probably acts as a donor of SH compounds essential for an unknown process, rather than as a reducing agent. The inhibition of hemolysin formation by 3 amino acid analogues phenylserine, phenylglycine and acetyl-pro-line is reversed to a large extent by sulfhydryl compounds. Thus 0.002 M of cysteine reverses to a large extent the inhibition of hemolysin formation caused by as much as 0.1 M of phenylserine.