The genes encoded by the RD1 locus are known to be important for intracellular survival of pathogenic mycobacterium, however their role in counteracting host defense is not known. I hypothesize that RD1 is involved in counteracting host oxidative response by secreting ROI-neutralizing enzymes such as catalase and superoxide dismutase. In support of this hypothesis I have shown that M. marinum RD1 mutants are more sensitive than wild type to ROIs both in vitro and in BMDM. Western blot analysis on the KatG protein levels within the bacterial cells grown in 7H9 rich medium demonstrated that wild type bacteria produced significantly higher amount of KatG than the RD1 mutants. When the bacteria were exposed for 2 hours to H2O2, wild type showed a significant reduction of the KatG level, while the RD1 mutants maintained constant levels of KatG, suggesting that RD1 genes might be involved in the secretion of KatG upon exposure to H2O2. These results together demonstrate an important unknown function of RD1 in resistance to ROIs.