Progressive heat inactivation of N.D.V. at 54–58° C. for 15 min. is accompanied by parallel destruction of its haemagglutinating activity. Progressive heat inactivation of Lee virus at 50–54° C. for 15 min. is accompanied by parallel loss of eluting power and ‘enzymic’ action on ‘Francis inhibitor’. Formalin in low concentrations rapidly inactivates both N.D.V. and Lee virus without appreciable impairment of their haemagglutinating activity or ‘enzymic’ function. Higher formalin concentrations acting over several days may, however, produce a slow deterioration of the haemagglutinin of both viruses. It is suggested that heat destroys the infectivity of N.D.V. and influenza virus primarily by abolishing their haemagglutinating ‘enzyme’. The possibility of the existence of independent virus components, all of which are essential for full infectivity of these viruses, is discussed.