Bronchial mucosal biopsy specimens were obtained during fibreoptic bronchoscopy in 30 patients receiving a new oral cephalosporin antibiotic, cefaclor (10 had 250 mg, 10 had 500 mg, and 10 had 1000 mg every eight hours). In 10 patients (from all dosage groups) cefaclor was undetectable in the bronchial mucosa but in every case the serum concentration was low, suggesting incomplete absorption. The mean (SD) bronchial mucosal concentration after 250 mg was 3.78 (1.77) micrograms/g (range 2.1-5.8 micrograms/g, n = 4), after 500 mg 4.43 (2.04) micrograms/g (range 2.0-7.1 micrograms/g, n = 8), and after 1000 mg 7.73 (2.76) micrograms/g (range 5.0-12.7 micrograms/g, n = 6). A significantly higher concentration in the bronchial mucosa was achieved with 1000 mg than with 250 mg (p less than 0.05) or 500 mg (p less than 0.025). These concentrations should be effective against Streptococcus pneumoniae, most strains being inhibited below 1.0 microgram/ml. The concentrations were within one dilution of the minimal inhibitory concentration for Haemophilus influenzae, most strains being inhibited below 4.0 micrograms/ml. Some strains of H influenzae will not be inhibited by the concentrations of cefaclor found in the bronchial mucosa, particularly those that are ampicillin resistant.