Acquired resistance to rifampicin by Mycobacterium kansasii.

Two patients with Mycobacterium kansasii infection of the lung had organisms sensitive to rifampicin. Following treatment, essentially with rifampicin alone, the patients began to excrete organisms completely resistant to rifampicin. The ability of M. kansasii to acquire resistance to rifampicin during treatment has been clearly demonstrated. This reinforces the need to treat this infection with an adequate multiple drug regimen.