The susceptibility of different species of mycobacteria, other than M. tuberculosis, to a range of cephalosporins and to amikacin was studied. Susceptibility patterns varied with species. M. fortuitum, M. marinum and M. szulgai were the most susceptible species to amikacin and to cefoxitin, where as M. kansasii, M. scrofulaceum and MAIS complex the most resistant. Cefoxitin appears to be much more active against mycobacteria than the other cephalosporins used in this study. Most of the cefoxitin-sensitive mycobacteria were inhibited by concentrations which can be easily attained in serum on standard dosage schedules.