Prevention of measles and rubella can be obtained by a single administration of bivalent vaccine which can be added to the trivalent vaccine already practice in the USA since 1970 and in the Scandinavian Countries since 1982. We evaluated the immunological response and the clinical reactions to a bivalent vaccine against measles and rubella by the study of a control group including 29 children (17 females and 12 males) aged from a minimum of 15 months to a maximum of 9 years and 4 months. All the children who resulted not to be immune to the 2 viruses by serologic tests showed a rise in the antibody titer with 100% seroconversion, whereas in those proving to be already immune to one of the 2 viruses, a persistent antibody titer or an increase was revealed in all except one. Neither general nor local clinical reactions were observed except for two children who showed feverish measles-like reactions 8 days after the vaccine administration. These results suggest that this bivalent vaccine is highly effective, without negative effects. Furthermore associated with trivalent vaccines, it offers larger immunization opportunity mainly in children who have already been affected by mumps.