Bacteriostatic and bactericidal activities of rifampicin, ethambutol, enviomycin and streptomycin on Mycobacterium avium-Mycobacterium intracellulare complex (MAI complex) strains were determined. The susceptibility testings were made in Ogawa egg medium, and minimal inhibitory concentrations (MICs) were determined as the lowest concentration of drugs, in which the growth of 20 to 100 colony-forming units was completely inhibited. The MICs correspond to the MICs that can inhibit the growth of 95 to 99% of bacterial population. Bactericidal activity was determined in a modified Dubos liquid medium (1.3 g of Dubos TB Broth Base were dissolved in 180 ml of distilled water and this solution was supplemented by 20 ml of bovine serum). A one ml-sample of bacterial suspensions (10 mg wet weight per ml) was added to 9 ml of the Dubos liquid medium, and the medium was incubated at 37 degrees C for 0, 1, 3 and 7 days under shaking condition (56 strokes per minute; 8 cm amplitude). The bactericidal activity was measured as the number of colony-forming units contained in a 0.02 ml-sample of the medium. The bactericidal activities of rifampicin and ethambutol were weak or absent even in strains 13008 and 13016, which were very susceptible to all four drugs. However, the bactericidal activities of streptomycin and enviomycin could be observed in these strains.(ABSTRACT TRUNCATED AT 250 WORDS)