Two methods for testing the susceptibility of Mycobacterium tuberculosis to rifapentine have been developed: the agar proportion method and the radiometric BACTEC technique. A critical concentration of 0.5 microg of rifapentine per ml is proposed for both methods since it provides a reliable means of distinguishing between susceptible and resistant M. tuberculosis isolates. It is recommended that two quality control M. tuberculosis strains be used at the introduction of these tests in a clinical laboratory: one that is pansusceptible (H37Rv) and one that is resistant to rifapentine. The resistant strain can be obtained from the American Type Culture Collection, where it is deposited under the number ATCC 700457.