The isolation of mycobacteria from municipal and hospital water supplies prompted an investigation of the susceptibility of environmental and clinical isolates of mycobacteria other than Mycobacterium tuberculosis and Mycobacterium bovis to free chlorine. Experiments revealed that free chlorine concentrations of 1.0 mg/l eliminated 100,000 c.f.u. of the mycobacterial strains tested within 8 hours of exposure, whereas a concentration of 0.15 mg/l had virtually no bacteriocidal effect. Free chlorine residual levels of 0.1 mg/l or less, depending on the water temperature within Boston, suggest that current disinfection procedures may not be adequate for effective control of potentially pathogenic mycobacteria in public water supply systems serving a population with increased risk factors.