The antigenic properties of vaccine and wild type strains of measles virus were compared. Serum specimens from vaccinated persons, persons infected during the prevaccine era, or mice experimentally vaccinated with the hemagglutinin (H) protein from vaccine virus neutralized vaccine virus and a wild type measles virus from 1989 equally well. In contrast, serum specimens from patients with recent measles virus infection and mice experimentally vaccinated with the H protein from the wild type virus from 1989 neutralized wild type virus with titers 4-8 times higher than those to vaccine virus. Several H protein-specific monoclonal antibodies could differentially recognize vaccine or wild type virus. These data show that the H proteins of the recent wild type viruses contain both conserved and new or modified antigenic determinants and are consistent with previous studies that described genetic drift in the H proteins of recent wild type viruses.