In a cross-over study, 10 healthy adult volunteers received 1 g cefaclor and 1 g cephalexin orally 1 hour after a standard breakfast. Serum levels of cefaclor rose somewhat faster and were higher than those of cephalexin in the first 60 minutes. At the end of the second hour the serum concentration of cephalexin was 50--100% higher. The area under the serum level curve averaged 45 hr-micrograms/ml for cefaclor and 60 hr-micrograms/ml for cephalexin. Calculated pharmacokinetic constants indicate that the absorption rate of cefaclor was nearly the same as that of cephalexin. The serum half-life of cefaclor was 1 hr. The urine recovery of cefaclor for 9 hours averaged 65%, whereas cephalexin resulted in a urine recovery of 85%. To evaluate sputum levels, 15 adult patients with bronchial carcinoma and secondary bronchitis or pneumonia were treated with 0.5 g of cefaclor orally 4 times daily. The mean individual sputum levels were 0.44 micrograms/ml (after the first dose) and 0.54 micrograms/ml (after repeated administration). In 10 other patients, after a single 1 g dose of cefaclor mean sputum levels after 1 hour and 3 hours doubled, whereas peak concentrations in sputum after 2 hours did not differ significantly. These results suggest that cefaclor will be useful in respiratory tract infections caused by sensitive bacteria.