Ninety-nine strains of streptococci were isolated from 97 cases of pyogenic infections, most of which involved the teeth. Physiological and serological tests were performed on these streptococci and on 37 strains of streptococci from culture collections. The results were used for a numerical classification. Seventy-nine of the strains isolated from patients formed a cluster with Streptococcus milleri and group-F reference strains, and were therefore considered as streptococci resembling S. milleri. By the use of an antiserum prepared against strain Z3, protein antigens were demonstrated in acid extracts of 65% of the strains of S. milleri. These antigens were in only five strains not included in the S. milleri cluster.