Four oral penicillin V regimens were compared for the ability to prevent Streptococcus sanguis infection of experimentally induced valvular heart lesions in rabbits. Challenge doses of 10(4), 10(6), and 10(8) CFU of a penicillin-susceptible strain of S. sanguis were used in this study. Measured by recovery of test organisms from endocardial lesions, the lowest-concentration inoculum was infective for 53% of the recipients; the higher-concentration inocula were infective for all recipients. A single-oral-dose penicillin V regimen (36 mg/kg of body weight) prevented endocarditis when rabbits were challenged with 10(4) CFU, but protection diminished with increasing inoculum concentrations. In contrast, addition of a second penicillin V dose (18 mg/kg of body weight) administered with a 7-h interval between doses achieved fully effective prophylaxis against even the highest inoculum tested (10(8) CFU). A repeated set of experiments in which half the dose of penicillin V was administered showed significantly reduced protection against S. sanguis endocarditis.