Due to the growing concern regarding risks to immunocompromised customers, ithas become necessary for utilities to focus on a variety of waterborne pathogens not previously examined. Members of the M. avium complex (M. avium and M. intracellulare) are opportunistic pathogens that cause debilitating and life-threatening infections in 25-50% of patients with acquired immunodeficiency syndrome (AIDS). This study focuses on the occurrence of M. avium complex (MAC) in drinking water systems and the methods used for the recovery of these organisms. The project included an 18-month full-scale study to evaluate the possible occurrence of MAC. One portion of the study was designed to collect raw water, plant effluent water and distributed water MAC samples from eight full-scale study sites. Biofilm samples were also collected using aged water meters from 26 different sites. Another portion of the study utilized a pilot system to evaluate the impact of pipe materials, disinfectants, and water temperatures on the survival of MAC. Includes 4 references, table.