A modification of an immunofluorescence method previously used to study the in-vitro antimicrobial susceptibilities of Chlamydia trachomatis was used to investigate the activity of seven antimicrobials against a strain of C. pneumoniae. Our results differed from those obtained by other workers, so we modified our original method and repeated the study. Adding antimicrobial to pre-infected cells gave higher MICs and MLCs than when cells were infected in the presence of the antimicrobials, and this difference in methodology could account for the discrepancy between our results and those of others. Of the antimicrobials studied, clarithromycin and its 14-hydroxy metabolite were the most active agents; sparfloxacin was more active than ciprofloxacin, but no more active than more conventional antichlamydial agents.