Untoward reactions induced by antimicrobial chemotherapeutic agents have had an increasing frequence in recent years. Lung and respiratory tract reactions are generally underestimated in comparison with those affecting other anatomical areas. The pathogenic mechanisms are reviewed under four headings, 1) toxic effects, 2) allergic reactions, 3) damage due to the antibacterial activity itself, and 4) lesions due to different causes. The various chemotherapeutic agents involved are thus mentioned in connection with these mechanisms.