The antimycobacterial spectrum of clarithromycin, a new macrolide drug, was screened against 26 strains belonging to 18 species of mycobacteria by determining radiometric MICs by Bactec methodology at three different pHs, i.e., pH 6.0, 6.8, and 7.4. The MICs obtained at pH 7.4 were 1 to 2 or more dilutions lower in some of the species than those obtained at pH 6.8, whereas they were significantly higher at pH 6.0. The numbers of species for which MICs were below the Cmax levels of clarithromycin (i.e., 4 micrograms/ml) at different pHs were 8 of 18 species at pH 6.0, 13 of 18 species at pH 6.8, and 14 of 18 species at pH 7.4. The drug possessed promising activity against such potential pathogens as M. avium, M. scrofulaceum, M. szulgai, M. malmoense, M. xenopi, M. marinum, and M. kansasii and rare pathogens like M. chelonei.