After a 500-mg dose of cefaclor, the mean peak plasma level was 12.4 μg/ml and after a 250-mg dose it was 5 μg/ml in normal volunteers. Food intake significantly reduced absorption. Probenecid prolonged plasma levels. Mean plasma half-life in normal volunteers was 0.8 h. Only about 50% of the dose was excreted in the urine within 4 h in normal volunteers. Plasma half-life in patients with renal insufficiency was only about 3 h, which suggests that cefaclor may be eliminated by nonrenal mechanisms in humans. Urinary levels of cefaclor were adequate to inhibit susceptible pathogens even in patients with moderately severe renal failure. Plasma half-life during hemodialysis was 2.1 h and rose to 2.8 h after dialysis.