145 consecutive patients scheduled for a cardiac catheterization were enrolled in a prospective double-blind randomized study to determine the incidence of bacteremia during and following catheterization and the usefulness of antibiotic prophylaxis with cefazolin versus placebo during this procedure. Four blood cultures were taken from each patient to evaluate the incidence of bacteremia vs. contamination. 15 of 296 (5.07%) and 11 of 284 (3.8%) cultures yielded bacteria in the placebo group (74 patients) and in the cefazolin-group (71 patients) respectively. Statistical analysis revealed no significant difference between the groups. These results, together with the spectrum of the organisms isolated and the clinical and laboratory findings, suggest that the isolation of bacteria was due rather to contamination than the result of bacteremia. It is concluded that antibiotic prophylaxis in patients undergoing cardiac catheterization is unnecessary and not indicated.