Further-attenuated strains of measles virus may be distinguished from virulent strains by the following characteristics. Unlike virulent strains, attenuated strains of measles virus can be propagated in chick embryo fibroblast cultures, induce production of interferon in tissue culture cells, and produce plaques that can be distinguished from those produced by virulent strains. Unlike virulent measles virus, attenuated strains induce an inapparent infection in ∼85% of vaccinees. Symptoms such as fever, cough and rash, if present, are usually transient. Bacterial and central nervous system complications associated with natural measles infection are extremely rare after immunization. A 16-year, prospective study of immunologic response of 47 children who had natural measles and of 70 children immunized with live, further-attenuated measles vaccine revealed (1) that all 47 children with natural measles infection had high titers of hemagglutination-inhibiting (HAl) antibody (⩾1:64) one month after vaccination and 15% had low titers (1:2–1:4) 16 years after vaccination; and (2) that all 70 children who received live, further-attenuated measles vaccine had high titers of HAl antibody (⩾1:64) one month after vaccination, but in 36% of the group the titers declined to <1:8 16 years later. Neutralizing antibody was detectable in convalescent sera that had lost detectable HAl antibody. Reimmunization of children with undetectable HAl antibody induced a classic booster response. These prospective immunologic studies confirmed that immunity persists after immunization as well as after natural measles infection.