We describe the microbiological and morphological characteristics of 151 strains of Streptococcus milleri isolated during the course of routine bacteriological investigations. Although these strains formed a fairly heterogeneous group, several constant features were identified which typify the species. Strept. milleri emerged as a major cause of pyogenic infection, clinical disease being characterized by localized collections of pus in almost every organ system. Bacteraemia due to Strept. milleri was a significant indicator of the presence of an occult abscess. Endocarditis was rare. The penicillins or erythromycin remain the drugs of choice for treating infections caused by this organism