BACKGROUND Mycobacterium avium complex (MAC) is a major cause of infection in immunocompromised patients. MAC possesses an enzyme that reduces potassium tellurite in less than 3 days and results in formation of a black precipitate. The objective of this study was to determine whether reduction of potassium tellurite by mycobacteria can be used as a means of testing the susceptibility of MAC to clarithromycin. METHODS Minimum inhibitory concentrations (MICs) for 104 clinical isolates of MAC were determined by the tellurite method and compared with those tested by a recommended microdilution method. Microdilution breakpoints were used for interpretation of susceptibility. MIC of less than 8 microg/mL was considered as susceptible, and MIC of greater than or equal to 8 microg/mL was resistant. RESULTS There was agreement within a 2-fold dilution between MICs for 89% of isolates. Of the 53 isolates that had discrepant MICs by the two methods, 70% had higher MICs by the tellurite method. When the MICs were classified into interpretive categories, there was 100% agreement by the two methods. The MIC tested by the tellurite method was available within 5 days. CONCLUSIONS These data suggest that use of potassium tellurite is a more rapid, reliable, and inexpensive method of testing the susceptibility of MAC to clarithromycin.