In 206 cases of bacterial endocarditis with positive blood cultures, 95% of 789 blood cultures were positive for the causative microorganisms. In streptococcal endocarditis, the first blood culture was positive in 96% of the cases, and one of the first two blood cultures was positive in 98% of the cases. In endocarditis caused by microorganisms other than streptococci, the first blood culture was positive in 82% of the cases, and one of the first two blood cultures was positive in all of the cases. The incidence of positive cultures was slightly reduced in those cases in which the patient had received an antimicrobial agent within two weeks prior to blood culture. The bacteremia in the 206 cases of bacterial endocarditis was persistent and of low magnitude.