Objectives: This study aimed to provide basic pharmacodynamic information for key antibiotics used to treat Mycobacterium avium and Mycobacterium xenopi pulmonary disease. Methods: M. avium subspecies hominissuis IWGMT49 and M. xenopi ATCC 19250 type strains were used; the MICs of clarithromycin, amikacin and moxifloxacin were determined by broth microdilution. Time-kill assays were performed, exposing bacteria to 2-fold concentrations from 0.062xto 32xthe MIC at 37 degrees C for 240 h for M. avium or 42 days for M. xenopi. The sigmoid maximum effect (E-max) model was fitted to the time-kill curve data. Results: Maximum killing of M. avium by amikacin was obtained between 24 and 120 h (0.0180 h(-1)) and was faster and higher than with clarithromycin (0.0109 h(-1)); however, regrowth and amikacin-resistant mutants were observed. Killing rates for M. xenopi were higher, 0.1533 h(-1) for clarithromycin and 0.1385 h(-1) for moxifloxacin, yet required 42 days. There were no significant differences between the Hill's slopes determined for all of the antibiotics tested against M. avium or M. xenopi (P = 0.9663 and P = 0.0844, respectively). Conclusions: The killing effect of amikacin and clarithromycin on M. avium subspecies hominissuis was low, although amikacin activity was higher than that of clarithromycin, supporting its role in a combined therapy. Clarithromycin and moxifloxacin may have similar activity within treatment regimens for M. xenopi disease. Future studies of in vitro and in vivo pharmacokinetic/pharmacodynamic interactions are needed to improve the current regimens to treat these two important slowly growing mycobacteria in pulmonary disease.