The efficacy of dapsone against Mycobacterium avium, M. intracellulare, M. kansasii, M. fortuitum and M. tuberculosis was determined by disc elution in agar. The minimal inhibitory concentration at which 90% of the colony forming units were inhibited (90MIC) by dapsone was 8 mg/l for M. avium, M. intracellulare and M. kansasii and greater than or equal to 32 mg/l for M. tuberculosis and M. fortuitum. The 90MICs were confirmed by culturing these test organisms with dapsone in the BACTEC 460 system. Reduction of the 90MIC from 8 mg/l to 2 mg/l was observed by combining dapsone with either or both of the potentiators tested. The dapsone-potentiator combinations had no effect on the bacteria with 90MICs of greater than or equal to 32 mg/l. The clinical relevance of this drug combination against M. avium complex infections remains to be evaluated.