Reports have shown that oleanolic acid (OA), a triterpenoid, exists widely in food, medicinal herbs and other plants, and that it has antimycobacterial activity against the Mycobacterium tuberculosis strain H37Rv (ATCC 27294). In this study it was found that OA had antimycobacterial properties against eight clinical isolates of M. tuberculosis and that the MICs of OA against drug-sensitive and drug-resistant isolates were 50-100 and 100-200 microg ml(-1), respectively. The combination of OA with isoniazid (INH), rifampicin (RMP) or ethambutol (EMB) showed favourable synergistic antimycobacterial effects against six drug-resistant strains, with fractional inhibitory concentration indices of 0.121-0.347, 0.113-0.168 and 0.093-0.266, respectively. The combination treatments of OA/INH, OA/RMP and OA/EMB displayed either a synergistic interaction or did not show any interaction against two drug-sensitive strains. No antagonism resulting from the OA/INH, OA/RMP or OA/EMB combination was observed for any of the strains tested. OA exhibited a relatively low cytotoxicity in Vero cells. These results indicate that OA may serve as a promising lead compound for future antimycobacterial drug development.