The penetration of an antibacterial agent into the bronchial secretions is a crucial factor in determining its clinical efficacy in the treatment of bacterial respiratory infections. Roxithromycin is a novel macrolide compound active against the most frequent respiratory pathogens. Following administration of 150 mg p.o., we observed a prompt penetration of the compound into bronchial secretions of critically ill patients. Elevated concentrations above the MICs of the commonest susceptible pathogens are reached and maintained until the next administration.