In a randomized, single-blind trial, ceftibuten in doses of 200 mg and 300 mg administered b.i.d., was compared with cefaclor 500 mg t.i.d. in acute lower respiratory tract infections. A total 545 patients were enrolled, of which 263 were evaluable for efficacy. All patients were adults with a diagnosis of either bacterial pneumonia or bronchitis. The infective organism was eliminated in 83% of the patients in the ceftibuten 200-mg b.i.d. treatment group and in 85% of patients in the 300-mg b.i.d. treatment group. The organisms were eliminated in 79% of cefaclor-treated patients. Satisfactory clinical responses were obtained in 91% of patients in the ceftibuten 200-mg b.i.d. treatment group and in 92% of patients in the ceftibuten 300-mg b.i.d. group. Satisfactory clinical responses were obtained in 91% of cefaclor-treated patients. Predominant pathogens isolated were Streptococcus pneumoniae, Haemophilus influenzae, Moraxella (Branhamella) catarrhalis, and strains of Enterobacteriaceae. Adverse experiences reported were similar for the ceftibuten and cefaclor treatment groups. Gastrointestinal side effects occurred in 6% of patients treated with ceftibuten 200 mg BID, 9% in those treated with 300 mg BID, and 7% of cefaclor-treated patients. Ceftibuten 200 and 300 mg twice daily was as effective as cefaclor bacteriologically and clinically in the treatment of lower respiratory tract infections.