There is an obvious interest in a combined meningococcus-measles vaccine since the two diseases are widespread and serious in Third World countries among children under five years of age. The purpose of our study was to show the safety and effectiveness of such a combined preparation. The study covered 110 children between 8 months and 4 years of age who were followed systematically in a maternal child health center in the Paris area. Only 93 of them were checked before and after the immunization. The serologic titrations by the hemagglutination assay (IHA) for measles, and by radioimmunological assay (RIA) for meningococcus A and C showed that the Schwarz strain measles vaccine combined with meningococcus A or the association A+C does not interfere with the increase of A or C titers. 100% of the children showed a seroconversion equal to or less than 2 micrograms per ml, in the case of meningococcus A, as well as for C, regardless of age. Furthermore, 88% of the subjects showed a titer greater than or equal to 4 micrograms for the meningococcus A and 79% for C. On the other hand, meningococcus A or the association A+C seem to depress measles vaccine activity. Nevertheless, more than 80% of the children tested showed seroconversion when the measles vaccine was combined with meningococcus A, and only 69% when combined with meningococcus A and C.