A method for the speciation of viridans streptococci (non-groupable) is described. The major identification criteria are based on the reactions to a series of biochemical tests, including acid production in lactose, inulin, raffinose, mannitol and sorbitol, hydrolysis of arginine, esculin and Na hippurate, and production of polysaccharides in 5% sucrose media. A total of 450 strains was isolated from blood cultures, 183 of which were from confirmed cases of subacute endocarditis. The latter were identified as follows (%): Streptococcus sanguis I (25.7), S. mitis (19.7), S. sanguis II (19.7), S. mutans (17.5), S. milleri (12), S. morbillorium (3.2) and S. salivarius (2.2). Susceptibility to antibiotics was studied for 129 of these strains: 68% were susceptible to all drugs tested, 20% were resistant only to tetracycline, 4% only to penicillin (MIC = 0.5-4 micrograms ml-1) and 8% were multiply resistant (tetracycline, macrolides and related drugs, chloramphenicol, penicillin, high-level resistance to kanamycin and/or streptomycin [MIC = 1000-80.00 micrograms ml-1].