The diffusion of cephradine into the pericardial exudate was measured in 19 patients undergoing heart surgery. Every patient received 2 grams of cephradine (=30 mg/kg) during a 20 minute infusion period, before, during, and after the operation, and than at intervals of 6 hours, together 8 grams per day. The highest mean serum level, 10 minutes after the end of the first injection was 150 microgram/ml and after repeated dosages 102.5 microgram/ml, and at the end of the infusion interval 3.2 microgram/ml in the mean. At the time of opening the pericardium the concentration of the substance in the pericardial exudate, 15, 30, 45 and 60 minutes after finishing the first injection, varied from 4.1 to 38.6 microgram/ml in the average. The postoperative maximum levels were higher than 50 microgram/ml and 6 hours after the respective administrations the levels varied between 12.83 and 15.77 microgram/ml in each case. These high concentrations of cephradine in the pericardial exudate could not be attributed to seepage of blood into the pericardium.-At specific "check points" bacteriological materials were taken. Out of 1168 specimens only 6.7% were bacteriological positive. Drains in wounds, the pleura or the pericardium were sterile in most cases. Intravenous catheters were contaminated only in 1%. However the contamination rate rises with period of storage. The results show that cephradine exhibits good diffusion into the exudate of pericardium. With the chosen dosage the concentrations of the antibiotic in the pericardial exudate are higher than the MIC values of most pathogen bacteria.