A method for the speciation of viridans streptococci (devoided of group antigens) is described. The major identification criteria are based on the reaction of a series of biochemical tests such as acid production in lactose, inuline, raffinose, mannitol and sorbitol, hydrolysis of arginine, esculin and Na hippurate, and production of polysaccharides in 5% sucrose media. A total of 460 strains were isolated from human specimens and identified as follows: 118 Streptococcus mitis, 102 S. sanguis II, 75 S. Sanguis I, 87 S. milleri (Streptococcus MG-intermedius), 28 S. mutans, 25 S. salivarius, 14 S. morbillorium, 2 S. uberis and 9 unspeciated. Susceptibility to antibiotics was studied for 318 strains: 63% of them were susceptible to all drugs tested; 37% of the strains were resistant to one or several antibiotics as follows: 34% to tetracycline, 8.5% to macrolides and related drugs, 5.3% to streptomycin and/or kanamycin (MIC greater than 2,000 micrograms/ml), 5% to penicillin (MIC = 1-4 micrograms/ml) and 4% to chloramphenicol.