Summary: Live measles virus obtained from tissue culture autolysates is shown to evoke the production of neutralizing antibody when given intradermally or intraperitoneally to rabbits and guinea pigs and intravenously to chickens. Virus injected intraperitoneally in chickens failed to stimulate antibody production, however, even when given in multiple doses. The kinetics of neutralizing antibody production in the guinea pig in response to primary and secondary injections of live virus resembles that seen with low levels of nonviable antigens. Heat or electron inactivated virus retains full antigenicity at exposure levels and times greatly in excess of those required for loss of viability. Inactivation of viral infectivity with formalin is more detrimental to antigenicity than the other methods studied.