A technique to determine the effects of combining antimycobacterial drugs in liquid medium employing the radiometric growth readings in the Bactec system was tested using 20 Mycobacterium avium complex strains. Ten of the strains had been isolated from children with lymphadenopathy and ten from adults with pulmonary disease. All isolates were resistant to streptomycin, rifampicin, isoniazid and ethambutol when tested with a conventional resistant ratio technique on Löwenstein-Jensen medium. Synergistic interactions were shown for the two-drug combinations streptomycin + ethambutol and ethambutol + rifampicin against all 20 strains. Good efficacy was also found for all three-drug combinations containing ethambutol. Thus, although most isolates of the Mycobacterium avium complex are resistant to first-line antituberculous drugs when tested individually, they are susceptible in vitro to certain combinations of these drugs. This rapid radiometric assay is an efficient means for detecting such synergy.