Rifampin is a potent antituberculous drug. In the treatment of drug-resistant tuberculosis it is highly effective provided it is given in combination with other drugs to which the patient's organisms are sensitive. Rifampin and ethambutol is a particularly powerful combination and will achieve almost 100% sputum conversion. It seems likely that rifampin will replace streptomycin, and ethambutol will replace PAS in first-treatment cases. Optimum first-line treatment will thus consist of rifampin, INH and ethambutol, with the probability of almost 100% success and the possibility also that the total duration of treatment may be considerably reduced. Rifampin is well tolerated but it may give rise to liver dysfunction and thrombocytopenia in a small proportion of patients. Patients treated with rifampin must be kept under close supervision because of the risk of side effects and, more important, because irregular treatment may lead to the development of rifampin-resistant organisms.