One hundred and twenty six strains of streptococci isolated from the bloodstreams of 123 hospital in-patients--55 with endocarditis--and 255 strains isolated from the gingival crevices of 66 volunteers were characterised. Species isolated were Streptococcus mitior, S. sanguis, undifferentiated viridans streptococci, S. salivarius, S. milleri, S. mutans, S. bovis and S. faecalis. There was no significant difference between the distribution of species in blood and in the gingival crevice. S. mitior and S. sanguis were found less frequently in blood than their presence in the gingival crevice would have suggested, whereas S. milleri was found more frequently. This may indicate that S. milleri is more invasive or that it enters the bloodstream from sources other than the gingival crevice.