Study results of passive natural immunity anti-measles, and immune response to measles vaccine from different strains in Mexican children under one year old, are described in this paper. Mexican infants in these studies ended their maternal immunity at 8-10 months of age. Edmonston-Zagreb (EZ) vaccine strain had higher seroconversion rates comparing to Schwarz strain. Dose response effect was more closely related to Schwarz strain than EZ, since difference in seroconversion rates with standard, medium and high doses were statistically significant. However geometric mean post-vaccination titers were higher in children vaccinated with Schwarz strain, than EZ receptors.