A virulent Mycobacterium avium strain, LR25, which carries 3 plasmids (18, 28, and 165 kb) and grows at 43°C was compared to its plasmid-free, avirulent segregant, strain LR163, to investigate the basis for the latter's inability to grow at 43°C. The failure of mid-log phase cultures of strain LR163 to grow at 43°C was dependent upon the presence of high levels of culture aeration. In addition, highly aerated mid-log phase cultures of strain LR163 failed to grow at 379C, By contrast, late-log phase cultures of strain LR163 were capable of growth when shifted to 43°9C under highly aerobie conditions. Mid-log phase cells of strain LR163 had 30% of the catalase activity of mid-log phase cells of strain LR25 and were more susceptible to hydrogen peroxide (0.08% w/v). Catalase activities of late-log, early-stationary, and stationary phase cells of strain LR163 were Significantly higher than mid-log phase cells. Catalase activity of strain LR25 was highest in cells of mid-log phase cultures, whereas the catalase activity of strain LR163 was highest in cells of stationary phase cultures. These data support the idea that plasmid-encoded genes influence M. avium catalase activity. / Master of Science