A study was conducted in 89 rabbits with experimental aortic valve endocarditis caused by three different strains of Staphylococcus aureus to determine whether there was a correlation between the peak serum bactericidal titer of the four drugs tested and the vegetation titer. After four days of therapy both the rabbits with and those without sterile vegetations had median peak bactericidal titers of 1 : 8. The mean vegetation titers did not correlate with the mean bactericidal titers. The serum bactericidal test does not measure the relative rate of killing of the bacteria by the drugs. Although the test remains clinically useful for documentation of bactericidal activity, the minimum level of activity necessary for the test to serve as a predictor of outcome remains to be defined.