An animal model was developed for studying macrolide-resistant Mycobacterium avium complex (MAC) and to measure the effect of ethambutol on resistance. MAC-infected beige mice were given clarithromycin daily; the frequency of clarithromycin-resistant MAC after 8 and 12 weeks was 10(-3) and 10(-2), respectively. Combined ethambutol plus clarithromycin did not increase anti-MAC activity, but clarithromycin-resistant MAC was less frequent (P < .05). The frequency of clarithromycin-resistant MAC in mice receiving the combination was significantly higher than that in untreated mice. These results are consistent with two human trials, which showed that adding ethambutol reduced the frequency of clarithromycin-resistant MAC. Results of the present study suggest that with an initially high level of MAC infection, the addition of ethambutol may only delay resistance. This mouse test system will be useful for investigating the influence of the level of MAC infection and the effect of other drugs on the frequency of resistant MAC.