To assay the efficiency of cephalothin prophylaxis in open-heart surgery, bacteriological examination of pressure-measurement units, intravenous catheter tips, and urine were made in 211 consecutive patients as well as blood cultures and sputum in suspected postoperative sepsis. Furthermore, cephalothin concentration in serum and tissue was determined in 12 consecutive adults with intact kidney function. Samples were taken before, during, and after the cardiopulmonary bypass, the tissue from the right atrium only before and after cardiopulmonary bypass. A high serum cephalothin level (80.04 +/- 23.35 microgram/ml) was measured 30 min after administration of 2 g cephalothin given as a 15-min-long i.v. infusion on induction of anesthesia. An antibiotic regimen - 4 X 2 g dose of cephalothin daily (first dose on induction of anesthesia) - provides a serum cephalothin level which is significantly higher than the cephalothin minimum inhibitory concentrations for most gram-positive organisms (0.475 microgram/ml) and so ensures an adequate antibiotic coverage throughout the surgical procedure and during the early postoperative phase of open-heart surgery.