Mycobacterial susceptibility testing is important for the management of patients with tuberculosis and those with disease caused by certain nontuberculous mycobacteria. To help standardize methods used in the clinical microbiology laboratory for testing susceptibility of mycobacteria, the National Committee for Clinical Laboratory Standards (NCCLS) recently updated NCCLS document M24-T (published in 1995), which is the tentative standard for antimycobacterial susceptibility testing of Mycobacterium tuberculosis. The second edition of the NCCLS tentative standard (document M24-T2) differs considerably from the initial document. It contains revised guidelines for the testing of M. tuberculosis complex and newly proposed guidelines for the testing of some nontuberculous mycobacteria, including the rapidly growing mycobacteria (Mycobacterium fortuitum group, Mycobacterium chelonae, and Mycobacterium abscessus), Mycobacterium avium complex, Mycobacterium kansasii, and Mycobacterium marinum, as well as Nocardia species and other aerobic actinomycetes. The recommendations for mycobacterial susceptibility testing that are outlined in NCCLS document M24-T2 are reviewed.