WR99210, a dihydrofolate reductase inhibitor, has promising in vitro activity against Mycobacterium avium complex (MAC). The in vitro activities of WR99210 alone and in combination with a fixed concentration of dapsone (0.5 microgram/ml) were evaluated against 35 clinical MAC isolates by a broth dilution method. The MIC at which 50% of isolates were inhibited (MIC50) and MIC90 of WR99210 alone were 2 and 8 micrograms/ml, respectively. The MIC50 and MIC90 of WR99210 in combination with dapsone were 0.25 and 4 micrograms/ml, respectively. Overall, 75% of the MAC isolates displayed enhanced susceptibility to the combination.