Microcystin-LR (MLR) has been identified in source and treated drinking waters in the U.S. and Canada. Chlorine is used by more than 95% of the medium and large water utilities and as a result would be easily accessible if CT values were developed for chlorination of MLR. Seven batch experiments were conducted to determine the interaction of MLR and free chlorine at 10C at pH values from 6 to 9 over 100 minutes of contact time. Samples were analyzed by enzyme-linked immunosorbent assay (ELISA). Inactivation of MLR increased as pH decreased and as the free chlorine concentration increased. Complete inactivation of MLR was not found, but concentrations near the detection limit of ELISA were observed in several cases. Includes 40 references, tables, figures.