We evaluated the effects of the combination of penicillin G and gentamicin against 10 penicillin-resistant bacteremic isolates of viridans group streptococci for which the MICs of penicillin were 4 to 64 micrograms/ml. In time-kill studies, the combination resulted in more killing of eight isolates for which the MICs of penicillin were from 8 to 64 micrograms/ml than any of the antimicrobial agents tested alone. In general, clearly enhanced antimicrobial activity was observed with the combination.