Children immunized with inactivated (Tween 80 and diethyl ether, TE) measles vaccine, attenuated live vaccine, or a combination of the two types were studied with regard to the production of hemagglutination-inhibiting (HAl), hemolysis-inhibiting (HLI), and nucleocapsid complement-fixing (CF) antibodies. For identification of antibodies to envelope components distinct from hemagglutinin, HLI tests were done both before and after removal of HAl antibodies with TE-treated virus antigen. Immunization with three or four doses of TE vaccine led to the production of HAl antibodies and, in some cases, of nucleocapsid CF antibodies. Non-HAl HLI antibodies were not detected. In contrast, live vaccine caused the appearance of non-HAl HLI antibodies and, in the majority of cases, nucleocapsid CF antibodies. After combined immunization with TE vaccine and attenuated vaccine, non-HAl HLI antibodies were absent from 25 of 29 children. Most children exposed to wild measles virus after immunization with TE vaccine alone developed a poorer non-HAl HLI antibody response than children who had had normal measles. The latter infections resulted in a more pronounced non-HAl HLI and nucleocapsid CF antibody response than that observed after infections with vaccine virus. An even more accentuated response involving these antibodies was seen in patients with subacute sclerosing panencephalitis. Thus the failure ofinactivated measles vaccines to prevent infection may be due to the absence of an envelope component responsible for the production of non-HAl HLI antibodies.