Superinfections with resistant micro-organisms have recently become an increasing problem in hospital patients and are throught to be contributed to by inappropriate use of newer, broad-spectrum antibiotics. Erythromycin exhibits a more limited spectrum of antibacterial activity and was therefore re-evaluated in the treatment of acute chest infections in critically ill patients. Twenty-one patients were given erythromycin in a dose of 1.8 g per day by continuous intravenous infusion for up to five days. A satisfactory clinical response occurred in ten patients, in all of whom erythromycin-sensitive micro-organisms were isolated from sputum. An unsatisfactory response occurred in six patients, all associated with micro-organisms insensitive to erythromycin and requiring alternative antibiotic treatment. No undesirable side-effects were noted. It is concluded that erythromycin is a safe and useful agent in the routine treatment of acute chest infections in critically ill patients.