Cefazolin was added to dialysis fluid in nine patients with renal failure undergoing peritoneal dialysis with 2 liters/h per exchange. With 50 mg/liter (three patients), the first measurable mean serum concentration was 3.7 mug/ml (range, 2.4 to 4.9 mug/ml) after three exchanges and was 30.3 mug/ml (range, 13.3 to 44.3 mug/ml) after 24 exchanges. After 18 to 24 more exchanges without cefazolin, the mean serum concentration was 12.3 mug/ml (range, 3.8 to 24.6 mug/ml). The mean concentration in the dialysis outflow was 26.9 mug/ml (range, 2.5 to 58.5 mug/ml). With 150 mg/liter (six patients), the mean serum concentration was 5.2 mug/ml (range, 3.6 to 7.8 mug/ml) after the first exchange and 8.4 (range, 6.1 to 14.0 mug/ml), 15.0 (range, 8.2 to 23.5 mug/ml), and 71.9 (range, 26.2 to 142.1 mug/ml) mug/ml after the second, third, and twenty-fourth exchanges, respectively. After 17 to 24 more exchanges without cefazolin, the mean serum concentration was 38.2 mug/ml (range, 15.4 to 65.7 mug/ml). The mean concentration in the dialysis outflow was 71.4 mug/ml (range, 21.9 to 150.8 mug/ml). After 1 g of cefazolin was given intraperitoneally with no more dialysis, serum concentrations rose by a mean of 62.5 mug/ml (range, 18.9 to 107.8 mug/ml). The maximum rise occurred within 2 h with two-thirds of the rise occurring within 30 min. During the subsequent 22 h levels dropped to 65.4% of the peak.