Mycobacterium avium is a waterborne opportunistic pathogen included in the Drinking Water Contaminant Candidate List by the US Environmental Protection Agency (US EPA, 1998). M. avium can cause both pulmonary and non-pulmonary infections, primarily to patients with HIV and other immune-compromised individuals (Joseph et al., 1996). The main source of M. avium infection for AIDS patients is drinking water. DNA fingerprints of M. avium isolates from water to which AIDS patients were exposed were identical to those of the patients (von Reyn et al., 1994). Further, the high incidence of M. avium infections in AIDS patients in Finland correlated with high numbers of M. avium in drinking and environmental waters (Ristola et al., 1999). Mechanisms contributing to the survival of M. avium inside a distribution system are not well known. The bacteria could be partially protected against the action of disinfectants by embedding into biofilms and suspended particles. The disinfectant might be absent, or present at lower concentrations compared to that in the bulk water, as a result of demand exerted by biofilm or particle organic matter. Furthermore, exposure to low concentrations of the disinfectant hydrogen peroxide has been shown to trigger an adaptive response referred to as the SOS response, with both Aeromonas hydrophila (Landre et al., 2000) and Escherichia coli (Imlay and Linn, 1987). An SOS response has also been observed for the inactivation of E. coli with monochloramine at relatively low concentration (Dunahee and Marinas, 2000). An analysis of the results presented by Pelletier et al. (1991) indicated that an adaptive response might also occur for the inactivation of M. avium with monochloramine. The occurrence of an adaptive response could be a major mechanism responsible for the enhanced resistance of M. avium to disinfectants in drinking water distribution systems. One of the goals of the present study is to assess if the adaptive response occurs during the inactivation of M. avium with both free and combined chlorine. Experiments designed to elucidate the occurrence of adaptive response during the inactivation of M. avium with monochloramine are presented in this paper. Preparation of bacterial suspension is discussed, along with experiments performed to assess the effect of monochloramine concentration on the inactivation kinetics of M. avium in a 0.01-M PBS at pH 8. Viability was determined by the membrane filtration method using mixed cellulose membrane with a diameter of 47 mm and a nominal pore size of 0.1 um. Includes 10 references, figure.