A multicenter, randomized, double-blind, single-dummy placebo-controlled study is being undertaken by the Research Unit of the Royal New Zealand College of General Practitioners to compare the efficacy and tolerance of 150 mg twice daily roxithromycin with 250 mg three times daily cefaclor in the treatment of 250 general practice patients with acute lower respiratory tract infections (LRTIs). Interim analysis of 200 patients reveals no statistically significant differences in the study parameters. Of the patients on roxithromycin and cefaclor, 83% and 67%, respectively, had a moderate or severe illness. Based on efficacy criteria, 96% of roxithromycin recipients and 99% of cefaclor recipients had a satisfactory or improved response. On an intention-to-treat basis, this was reduced to 95% for both treatment groups. Sputum grading and semiquantitative culturing was performed according to NCCLS standards. The most common isolates in order were Haemophilus influenzae, Moraxella catarrhalis, and Streptococcus pneumoniae. Efficacy for bacteriologically evaluable cases was 87.5% for roxithromycin and 57% for cefaclor. Four patients on roxithromycin (3.9%) and 11 patients on cefaclor (11.3%) withdrew because of side effects probably or possibly related to the study treatment. The study is ongoing.