Military recruits with or without antibodies to mumps virus were immunized with live or formalin-inactivated mumps virus vaccines. Antibodies to the two major surface components of the virus, the hemagglutinin and the hemolysin, were identified separately. Immunization with the live vaccine did not change the humoral immunity in individuals with detectable antibodies. In seronegative recruits immunization with the live vaccine induced an antibody response against both the hemagglutinin and the hemolysin corresponding to the quality of the immune response after natural infection. In contrast, the formalin-inactivated vaccine only induced an antibody response against the hemagglutinin. This effect was found after immunization of both seronegative and seropositive individuals. The latter displayed a selective boosting of titers of hemagglutination-inhibiting antibody. These findings may have relevance for the interpretation of the short-lived immunity after the use of formalin-inactivated mumps vaccine without repeated boosting.