Amoxicillin prophylaxis against experimental endocarditis due to one nontolerant and two tolerant strains of streptococci was studied in rats. Single-dose amoxicillin protected against the two tolerant strains in animals challenged with the 90% infective dose (ID90), but protection diminished with increasing inoculum sizes. Protection against the nontolerant strain was successful with inocula that were 100- and 1,000-fold larger than the ID90. Close correlation existed between the speed of bacterial killing in vitro, the time of exposure to bactericidal levels in vivo, and the range of inocula against which prophylaxis was effective. Amoxicillin seemed to protect by at least two mechanisms. (1) When in vitro tests indicated adequate bacterial killing, protection was independent of the inoculum size and was probably conferred by bacterial killing. (2) When in vitro tests indicated bacterial inhibition but not killing, protection was inoculum-dependent and was probably mediated by inhibition of bacterial adherence.