The in-vitro activity of a new macrolide antibiotic CP-62,993 (Pfizer Ltd) was determined for 420 bacterial isolates, comprising 150 Haemophilus influenzae, 48 Branhamella catarrhalis, 50 Staphylococcus aureus, 50 coagulase negative staphylococci, 50 beta-haemolytic streptococci, 50 Streptococcus pneumoniae and 22 oral streptococci. CP-62,993 was compared with erythromycin and penicillin (ampicillin in the case of H. influenzae). The MICs of CP-62,993 were found to be lower than those of erythromycin for the two Gram-negative species tested: this was particularly marked in the case of H. influenzae where a ten-fold difference in the MIC50 was observed (CP-62,993, 0.25 mg/l, erythromycin 2 mg/l). In general MICs of CP-62,993 were two-fold higher than those of erythromycin for the Gram-positive species studied, with the exception of the oral streptococci which were equally susceptible (MIC50 0.03 mg/l) to both macrolides. Activity similar to that of erythromycin was observed against Str. pneumoniae (MIC values 0.015-0.12 mg/l), Staph. aureus (MIC 0.12-1 mg/l) and beta-haemolytic streptococci (MIC 0.06-4 mg/l). Incubation in a CO2 enriched atmosphere decreased activity of both erythromycin and CP-62,993. The effect was more marked for CP-62,993, the MIC50s of all groups of organisms being increased four-fold when they were incubated in the presence of 5 or 10% CO2.