A collection of 297 clinically documented 'Streptococcus milleri' strains, identified to the genotype level by 16S rRNA gene hydridisation, was screened for haemolysis of human and animal red blood cells. Forty-nine strains (65%) of the S. intermedius genotype displayed haemolysis restricted to human blood; they were named 'exclusive human haemolytic' (EHH) S. intermedius strains. The 26 remaining S, intermedius strains were named S, intermedius non-EHH strains. Quantitative studies on the haemolysis indicated that intermedilysin was the factor involved. The S, intermedius EHH strains represented the S, intermedius phenotype, whereas the S. intermedius non-EHH strains were phenotypically characteristic of S. constellatus. The complete 16S rRNA sequences of the S. intermedius EHH strains exhibited identity with S. intermedius strains ATCC 27335 (= NCDO 2227, NCTC 11324); the 16S rRNA sequences of the S. intermedius non-EHH strains were identical to S. constellatus strain ATCC 27823 (= NCDO 2226, NCTC 11325) except for positions 228 and 229 that carried an S. intermedius sequence signature. The 16S sequence similarities between the non-EHH strains and the S, constellatus and the S. intermedius type strains were 99.5% and 98.6%, respectively. Hybridisations of the complete 16S rRNA genes with oligonucleotide probes indicated a 16S rRNA homogeneity within the S. intermedius EHH and the non-EHH strains respectively. The S. intermedius EHH strains were isolated most frequently from infection- and abscess-related specimens. The present data emphasise the genetic variability within the S. constellatus species and redefine the S. intermedius species as a homogeneous group at the 16S rRNA level.