Mycobacterium sp. strain 12523 has a parasite-specific hydroxylation activity, which produce para-substituted phenols from various aromatic compounds. However, the activity is unstable and the reactions are inactivated within 24 h. In order to extend the reaction period, the factors that affected reaction stability were examined. The hydroxylation activity of the cells incubated in buffer was significantly stabilized by the inclusion of an inducer such as methyl ethyl ketone. It is suggested that a regulatory mechanism is involved in controlling the activity. This study resulted in the development of a convenient method to stabilize the hydroxylation activity, involving the addition of an inducer, such as acetone, to the reaction system. This method permitted the hydroxylation reaction to continue for more than 67 h.