We evaluated the activities of clarithromycin and azithromycin against 19 isolates of Streptococcus pneumoniae using a neutropenic lung infection model. The isolates included five susceptible isolates (clarithromycin and azithromycin MICs, </=0.12 micro g/ml), nine isolates exhibiting low-level, mefA-mediated resistance (clarithromycin and azithromycin MICs, 0.5 to 32 micro g/ml), and five isolates expressing high-level, ermB-mediated macrolide resistance (clarithromycin and azithromycin MICs, >/=64 micro g/ml). Infected mice were administered either saline (control), clarithromycin (4, 40, or 200 mg/kg of body weight twice daily or 200 mg/kg once daily), or azithromycin (4, 40, or 200 mg/kg once daily or 40 mg/kg twice daily) by oral gavage for 72 h. Mortality was assessed at regular intervals for 10 days, and survival in each group was compared to that of untreated controls. Animals infected with susceptible isolates demonstrated significant improvement in survival compared to the controls following treatment with either agent at doses of >/=40 mg/kg. In contrast, none of the regimens improved the survival of animals infected with isolates exhibiting high-level macrolide resistance. Among mice infected with strains expressing low-level resistance, significant improvement in survival compared to the controls was noted among isolates treated with clarithromycin at 40 (seven of nine isolates) and 200 (nine of nine isolates) mg/kg twice a day and with azithromycin at 40 (one of nine isolates) and 200 (three of nine isolates) mg/kg once a day. Animals infected with isolates of S. pneumoniae exhibiting low-level, mefA-mediated macrolide resistance responded to treatment with clarithromycin at rates similar to those observed among mice infected with fully susceptible isolates.