Catalase and superoxide dismutase, but not peroxidase activity was detected in cell-free extracts of Mycobacterium avium, M. intracellulare and M. scrofulaceum (MAIS). The M. scrofulaceum isolates had the highest catalase activity, while both M. avium and M. intracellulare had significantly lower activities. The percentage of catalase activity remaining, after exposing cell-free extracts from late log grown cells to 53°C for 50 minutes allowed differentiation among all three species. Polyacrylamide gel electrophoresis of crude extracts demonstrated two bands of catalase activity in both M. avium and M. intracellulare extracts and four bands of activity in M. scrofulaceum extracts. These bands differed in their susceptibility to heat inactivation and inhibition by 3-amino-1,2,4-triazole. M. scrofulaceum strains, but not M. avium and M. intracellulare, demonstrated extracellular catalase activity. The susceptibility to H₂O₂ of 6 M. avium strains, differing in catalase activity and cell permeability, were tested. At a concentration of 0.02% H₂O₂, all M. avium strains were resistant, while differences in susceptibility were seen at 0.08% H₂O₂. Strains of low extract catalase activity and high H₂O₂ permeability were most susceptible. The superoxide dismutase activities of the MAIS strains tested were similar and no species-specific differences could be discerned. Electrophoresis of crude extracts demonstrated a single band of activity for each of the MAIS strains. Extracellular superoxide dismutase activity was detected in four of six MAIS strains. The metal type of MAIS superoxide dismutase was indirectly determined by inactivation with KCN, NaN₃ and H₂O₂. / M.S.