Human oral isolates of Streptococcus intermedius, which were biochemically identified, have been classified into 5-Serogroups (I-V). Selecting some representive stock cultures together with the type strain ATCC 27335, comparisons were carried out between them in terms of cell extractable proteins, G + C mol% values, DNA-DNA homology and DNA restriction fragments. Since the taxon of S. intermedius has not been clarified, comparison was also made between the above strains and the type strains of the "Streptococcus anginosus-milleri Group", Streptococcus anginosus, "Streptococcus milleri" and Streptococcus constellatus, which were thought to be related to S. intermedius. 1. G + C mol% of S. intermedius ATCC 27335 was 38.0 +/- 0.28. The values of all the isolates tested ranged from 37.0 to 38.8. On the other hand, the values of the type strains of the "S. anginosus-milleri Group" were 38.0-38.8. 2. Under stringent conditions of DNA-DNA hybridization, all the isolates showed more than 66.7% homology with S. intermedius ATCC 27335, demonstrating that these strains belonged to the same species. On the other hand, S. anginosus ATCC 33397, "S. milleri" NCTC 11169 and S. constellatus ATCC 27823 hybridized at levels of 50%, 53% and 28%, respectively. Consequently, it was impossible to classify these strains as being the same species as S. intermedius. 3. When the DNA restriction fragments were compared by PAGE, the strains of Serogroups I and V showed the same pattern, respectively. The strains of Serogroups II and III, however, showed two different patterns, and these of Serogroup IV showed five different patterns. DNA fragments specific to S. intermedius could not be identified. 4. When cell extractable groups were compared by SDS-PAGE, the strains of Serogroups I and V showed identical patterns. The strains of Serogroup III showed 2 different patterns and those of Serogroup IV showed 5 patterns. These results reflected those of the DNA restriction analysis. However, the strains of Serogroup II showed the same protein pattern, although 2 different patterns were observed in DNA restriction fragments. Unique proteins, which were not detected in the proteins extracted from the type strains of the "S. anginosus-milleri Group", were observed in the cell proteins of S. intermedius strains. 5. The results of DNA-DNA hybridization and the existence of unique extractable proteins together with the chondroitinase productivity suggest that the strains of S. intermedius might represent a particular taxon among the strains of the "S. anginosus-milleri Group".(ABSTRACT TRUNCATED AT 400 WORDS)