The rate, type, and magnitude of bacteremia were studied in 56 patients undergoing dental cleaning with and without penicillin prophylaxis. Sixty-one percent of patients without penicillin prophylaxis were bacteremic 5 minutes following the procedure. Although a significant decrease in detectable bacteremia occurred in patients receiving penicillin prophylaxis, the recovery of streptococci was not significantly different in the two groups. Using the present sample of patients as a basis for statistical inference, the true rate of bacteremia in such patients could be between 41% and 79% with 95% certainty. The magnitude of bacteremia was low and positive quantitative pour plates occurred at 5 minutes and only in patients without penicillin prophylaxis. Of the 71 total bacterial isolates, 53 (74.6%) were anaerobes and 18 (25.4%) were aerobes. This study indicates that parenteral penicillin prophylaxis for dental cleaning decreased detectable bacteremia rates significantly and could be recommended for patients with valvular heart disease who are known to be vulnerable to endocarditis.