The number of cases with tuberculosis is again increasing in many countries, and recently several nosocomial outbreaks of multidrug-resistant tuberculosis have occurred in the United States. The number of patients with disseminated Mycobacterium avium complex (MAC) infections in AIDS population, and patients with MAC pulmonary disease unassociated with HIV seem to be also increasing. It takes at least 6 to 9 months for an initial treatment of active tuberculosis due to drug-sensitive strains with the standard regimen which includes isoniazid (INH) and rifampicin (RFP). Treatment for the diseases caused by drug-resistant M. tuberculosis and MAC is much more time-consuming and more toxic than for the diseases caused by drug-sensitive strains, and often unsuccessful. For the reasons described above, the developments of new agents with potent antimycobacterial activities are highly desired. The new agents should also be useful for treating patients who have acquired resistance to many of the currently available drugs. In this review the new antimycobacterial drugs are summarized. Some of them have already been used clinically, but many are still in experimental evaluations. 1) Rifamycin derivatives: rifabutin (RBT), KRM-1648 (KRM), rifapentin (RPT), FCE-22250, FCE-22807, CGP-7040, SPA-S-565 and other rifamycin derivatives. New rifamycin derivatives including RBT, KRM have increased in vitro antimycobacterial activities. RBT and KRM are much more active in vitro and in vivo than RFP against both M. tuberculosis and MAC. KRM seems to be more potent than RBT against MAC in experimental studies.(ABSTRACT TRUNCATED AT 250 WORDS)