The interaction between a new macrolide antibiotic, miocamycin, and theophylline was evaluated in a single cross-over study in 5 asthmatic children. Each patient received a single dose of theophylline (4.3 mg/kg) delivered in 15 min using a constant-rate infusion pump, immediately before and after a 10 day course of miocamycin 17.5 mg/kg b.d. The pharmacokinetics of theophylline were calculated for each phase of the study. The elimination rate constant (3.92 vs 3.74 h-1), the mean total body clearance (1.71 vs 1.8 ml X min X kg-1) and the mean apparent volume of distribution (0.57 vs 0.581 X kg-1) did not differ. The result can be explained by the inability of the antibiotic to form inactive cytochrome P-450 metabolite complexes which can interfere with the metabolism of theophylline. Thus, miocamycin can safely be administered to asthmatic children requiring theophylline treatment, when they have an infection due to susceptible pathogens.