Roxithromycin (RU 28965) is a new semi-synthetic derivative of erythromycin. Its antibacterial activity is of the same order as that of other macrolides, although its MIC's against Legionella pneumophila are lower. In double-blind comparative studies there was no significant difference in therapeutic effectiveness between roxithromycin and the reference antibiotics tested. However, this new macrolide exhibits exceptional pharmacological properties (prolonged half-life, excellent tissue penetration and intracellular activity), and it is very well tolerated both clinically and biochemically.