Intracellular bactericidal activities of the antituberculosis drugs rifampin and amikacin, as well as those of newly described drugs clarithromycin (a macrolide) and sparfloxacin (a difluoroquinolone), were assessed against three strains of the Mycobacterium avium complex (MAC) growing in two different in vitro macrophage systems, namely, mouse bone marrow-derived macrophages (BMMO) and human peripheral blood monocyte-derived macrophages (human MO). All the infected macrophages were fed reported C(max) concentrations of the drugs, i.e., 15 mug/ml for rifampin, 20 mug/ml for amikacin, 4 mug/ml for clarithromycin, and 1.5 mug/ml for sparfloxacin. Further potentiation of drug activity in the presence of C(max) level of ethambutol (6 mug/ml) during 9 days of intracellular growth (measured by lysing the macrophages and making bacterial counts) was assessed. Our results showed that all four drugs were active against the strains used in this study and that the addition of ethambutol (which had no significant intracellular activity against the bacteria in this system) further potentiated the bactericidal effect of the drugs. When the same drug combinations were tested at their sublethal concentrations by BACTEC(R) radiometric methodology, a good correlation between the drug enhancement data in extracellular and intracellular systems was found. We conclude that ethambutol may serve as an essential component in effective anti-M. avium chemotherapy and that the effective drug combinations may be routinely screened by the Bactec radiometric methodology.