The course of chlorination in neutrophilic granulocytes has been shown. The process of 36Cl incorporation occurs during and after the engulfment of bacteria by granulocytes. Incorported radioactivity was found in insoluble fractions. The myeloperoxidase obtained from neutrophils catalyzes chlorination of protein (bovine serum albumin) and bacteria (Staphylococcus epidermidis) in the presence of hydrogen peroxide and chloride. The products of chlorination are insoluble. Chlorination in neutrophils is inhibited by the iodide and myeloperoxidase inhibitors azide and cyanide. A quantitative method of determination of biological chlorination in cells has been devised.