Twenty clinical isolates of Mycobacterium avium-intracellulare were tested with amikacin and sulphamethoxazole for in-vitro susceptibilities. The MICs and MBCs of the former drug ranged from 8 to > 64 mg/L (median MIC: 64 mg/L, median MBC: > 64 mg/L). The MICs and MBCs of the latter drug were found to be > 256 mg/L. Each of eight patients with invasive pulmonary disease due to these organisms was treated with amikacin for six months and with cotrimoxazole (sulphamethoxazole-trimethoprim) for one year. Only one patient had sustained bacteriological conversion. Three patients showed a transient reduction of bacillary load during the period of amikacin administration. The rest all failed to show response. Thus sulphamethoxazole was found to have no activity against Mycobacterium avium-intracellulare, and amikacin has doubtful activity when used alone in treatment of M. avium-intracellulare infection.