The chloramine-T method (Greenwood, Hunter & Glover, 1963) has been used for a decade to produce high specific activity labelled proteins for use in radioimmunoassays. This procedure can result in a reduction of immunoreactivity and sometimes complete loss of biological activity. Modifications attempting to reduce the 'damage' have included cooling the reactants, reducing the amount of chloramine-T and adding the oxidant in small aliquots. Different methods have also been developed, including iodine distillation (Heideman, Levy, McGuire & Shipley, 1965), electrolysis (Rosa, Scassellati, Pennisi, Riccioni, Giagnoni & Giordani, 1964), lactoperoxidase (Thorell & Johansson, 1972) and conjugation with a radio-iodine containing acylating agent (Bolton & Hunter, 1973). Recently, Butt (1972) produced iodinated proteins with very little immunological or biological damage, releasing the iodine from the sodium iodide/protein solution with chlorine generated in an enclosed container. Different antigens require varying exposure times for optimum iodine incorporation and the method is cumbersome to operate.