The incidence of bacteremia during cardiac catheterization was studied in 122 adult patients and the protective effect of antibiotic prophylaxis was evaluated in treatment and control groups. Antibiotic prophylaxis did not decrease the incidence of transient bacteremia at cardiac catheterization. Staphylococci were the organisms most frequently isolated from blood cultures taken at catheterization and 50 per cent of the staphylococci were resistant to penicillin. A review of the literature suggests that endocarditis as a sequel to cardiac catheterization is a rare event, and may not justify the use of potentially sensitizing agents for prophylaxis. © 1969.