The in-vitro activity of penicillin and streptomycin against 48 strains of viridans streptococci isolated from patients with endocarditis was studied. All strains were inhibited by penicillin, up to 0.4 μg/ml, and streptomycin, up to 200 μg/ml. The combination of penicillin, 0.8 μg/ml, and streptomycin, 5 μg/ml, sterilized a 106 inoculum of 37 of the 48 strains. Penicillin, 1.6 μg/ml, killed only 1 strain, and none were killed by streptomycin, 10 μg/ml. Rate of killing was also increased for 45 of the 48 strains by the addition of streptomycin to penicillin. Over 100 patients with endocarditis caused by penicillin-sensitive strains of viridans streptococci (minimal inhibitory concentration, ≤ 0.1 μg/ml) have been treated at the New York Hospital with parenteral penicillin and streptomycin for 2 weeks, followed by parenteral penicillin alone for an additional 2 weeks, without relapse. Penicillin, in combination with streptomycin, appears to be the optimal therapy for viridans streptococcal endocarditis.