In vitro sensitivities to various drugs of a total of 106 strains of atypical mycobacteria were studied in modified Dubos Tween albumin liquid medium. Eight triple-drug combinations of antituberculous drugs were also evaluated in vitro for their potentiated activities. Minimal to moderate potentiation was demonstrated in the majority of the combinations. From the results, the triple-drug combination — including rifampin, one of three aminoglycosides (streptomycin, kanamycin, viomycin), and either ethionamide or ethambutol-might be recommended for Mycobacterium kansasii infections. Against Mycobacterium avium-intracellulare infections, rifampin-kanamycin-ethionamide or rifampin-kanamycin-ethambutol might be the choice if we were to select any triple-drug regimen. None of the triple-drug regimens thus far tested on M. avium-intracellulare were active enough to recommend fully for clinical use. In vivo experimental chemotherapy of murine infection with Mycobacterium intracellulare (TMC 1469) on a five-drug regimen, kanamycin-rifampin-cycloserine-ethambutol-ethionamide, showed a moderate therapeutic effect but the infection was not eradicated.