A description is given of the characters of certain streptococci which in small numbers are sometimes found in the milk of individual cows. Although the organisms possessed high haemolytic activity, they showed no evidence of pathogenic power, either for the cow's udder or for ordinary small laboratory animals. Not only are they different from the streptococci of bovine mastitis, but they are also to be distinguished from streptococci of the Str. pyogenes group which are occasionally found in milk. One of their principal characters is the formation on solid media of minute colonies, consisting of relatively minute cocci, which grow in broth as very short chains.

The fifty-eight strains studied could be divided into two groups on biochemical grounds: (a) those fermenting glucose and lactose with production of small amounts of acid, (b) those which ferment mannite and salicin as well. The former are referred to as “low-acid” strains and the latter as Str. infrequens, as they bear a resemblance to the organism given this name in Holman's classification. Fermentation of sorbite and trehalose occurred with streptococci of both groups.

The two groups could be distinguished from Str. pyogenes and the ordinary mastitis Streptococcus by direct agglutination. For distinguishing the two groups from each other, agglutinin absorption methods were required. It was then found that the low-acid Streptococcus possessed two antigens of which one only was represented in Str. infrequens.