Two groups of patients undergoing open-heart surgery were given prophylactic courses of antibiotic lasting five days. One group (61 patients) received a cephalosporin and the second (57 patients) received a combination of penicillin, flucloxacillin and streptomycin. The overall major infection rate was low (3-4%), particularly so in the sephalosporin group (1.6%). There was no increased nephrotoxic effect of the cephalosporin, and any nephrotoxic effect that was present was temporary and clinically unimportant. The major infecting organism in both groups was Staphylococcus albus (Staph. epidermidis). The efficiency, therefore, of any prophylactic regime which omits gentamicin, to which Staph. albus is usually sensitive, remains in doubt.