Forty-one strains of Streptococcus mutans (34 from blood specimens from patients with endocarditis and 7 from stock cultures) were tested for susceptibility to penicillin, ampicillin, methicillin, erythromycin, cephalothin, vancomycin, chloramphenicol, tetracycline, gentamicin, streptomycin, and kanamycin. Minimal inhibitory and bactericidal concentrations were determined by a broth microdilution procedure. Most of the strains were very susceptible to ampicillin, penicillin, and erythromycin, with most strains having minimal inhibitory concentrations of 0.08 mug/ml or less. Most of the strains were also susceptible to cephalothin, methicillin, chloramphenicol, tetracycline, and vancomycin. Gentamicin was the most effective aminoglycoside. The antimicrobial susceptibility patterns are similar to those of other viridans streptococci. S. mutans strains have proven to be difficult for some microbiologists to identify. But when organisms suggesting S. mutans are isolated from patients with endocarditis, they should be at least identified as nonenterococcal streptococci so that appropriate therapy can be initiated.