The growth and production of M protein by 18 strains of Streptococcus pyogenes of different M types were compared in semisynthetic media having either starch or Tween 80 as binders of oleic acid and either acid-hydrolysed casein or casitone as a source of amino acids. Most strains of M-type 12 showed substantial loss of M protein synthesis within several sequential transfers in the casein hydrolysate-Tween (CH-Tw) medium, but M protein synthesis was brough to its original concentration by a single transfer to semisynthetic medium to which 4% (v/v) horse serum or 50% (v/v) Todd Hewitt broth had been added. Six of eight strains of M-type 22 showed a complete loss of M antigen within one ortwo transfers in the CH-Tw medium. Re-initiation of M protein synthesis required the presence of both Todd Hewitt broth and horse serum. No evidence was found correlating loss of M protein to synthesis of proteinase since proteinase was not formed in the CH-Tw medium. Although the genetic loss of M protein synthesis did occur with one strain, the experimental results emphasized that variation in specific concentration of M protein or cessation of its synthesis resulted primarily from the absence of nutritional factors regulating its synthesis.