The in-vitro susceptibility of Mycoplasma pneumoniae to clarithromycin, a new macrolide, was compared with that to erythromycin. A broth microdilution assay was used to evaluate 30 clinical isolates collected over a 15 year period from four countries (United States, Australia, Denmark and Japan). Clarithromycin inhibited the growth of all M. pneumoniae strains at low concentrations (less than or equal to 0.008 mg/l) similarly to erythromycin (less than or equal to 0.008 mg/l). In 120 outpatients with radiographically confirmed community acquired pneumonia (mean age 46.2 years), M. pneumoniae was detected culturally and/or serologically by ELISA and immunoblotting in 13% of patients, thus confirming the continued importance of this organism as a respiratory pathogen. M. pneumoniae isolates from these patients were shown to be equally susceptible to clarithromycin (less than or equal to 0.008 mg/l) and erythromycin (less than or equal to 0.008 mg/l). Both clarithromycin and erythromycin were effective in the treatment of community-acquired pneumonia. There were no statistically significant or clinically important differences between the two treatment groups with respect to clinical or radiographic resolution of disease or improvement in signs and symptoms. While the number of clinically evaluable patients with M. pneumoniae infections was small, both macrolides seemed equally effective.