Strains of oral Streptococcus milleri group were compared with the type strains of Streptococcus anginosus, S. intermedius, and S. constellatus by DNA-DNA hybridization at 60 degrees C. Of the 29 strains tested, twelve, twelve, and two strains were closely related to S. anginosus ATCC 33397T, S. intermedius ATCC 27335T, and S. constellatus ATCC 27823T, respectively. Generally, the strains classified in the S. anginosus group in DNA homology were non-beta-haemolytic and belonged to API S. milleri II and biotype I (lactose fermenting), whereas the strains of S. constellatus group were beta-haemolytic and belonged to API S. milleri I and biotype II (lactose non-fermenting). The strains of the S. intermedius group were all non-beta-haemolytic and belonged to API S. milleri II and mostly biotype II (lactose fermenting). Furthermore, all except one strain of the S. anginosus group were Lancefield group A/serotype a (A/a), ungroupable/serotype b (-/b), C/c, -/d, -/e, F/f or G/k, whereas those of the S. constellatus group were F/-. The strains of the S. intermedius group were -/g, -/h, -/i, -/j, or -/-.