Thirty-five clinical isolates of Legionella species were tested against 7 antimicrobial agents using an agar dilution technique. Results obtained on charcoal-supplemented (BCYE) and charcoal-free agar (BSYE) were compared. On BCYE, the most active agent was rifampicin; the minimal inhibitory concentration inhibiting 90% of the strains (MIC90) was 0.008 mg/L. Imipenem was the next most active in vitro (MIC90 0.06 mg/L). The macrolide antibiotics and ciprofloxacin also inhibited the organisms at low concentrations (MIC90 < or = 2 mg/L). In general, MIC's obtained on BCYE agar were at least twofold higher than on BSYE agar except for that of imipenem. BSYE agar is a suitable alternative medium for susceptibility testing of most Legionella species. Erythromycin and rifampicin continue to demonstrate good in vitro activity against legionellae in Australia. On the basis of in vitro susceptibility tests, the other macrolides and ciprofloxacin are likely to be suitable alternatives for the treatment of legionellosis.