The effects of measles immunization on immune responses in infants and the roles of vaccine strain and age of immunization are not known. Eighty-eight children were immunized at 6 or 9 months of age with the Edmonston-Zagreb (EZ) or Schwarz (SW6, SW9) strain of measles vaccine. Children were studied before and 2 weeks and 3 months after immunization. Seroconversion was similar, but geometric mean neutralizing titers at 3 months differed by vaccine group: SW9, 1367 mIU/mL; SW6, 982; and EZ, 303 (P = .003). Mitogen-induced lymphoproliferation was decreased at 2 weeks in the SW9 group and at 3 months in all groups and was negatively correlated with measles antibody level at 3 months (r = −.387, P = .003). CD8 T cells, soluble CD8, neopterin, and β2-microglobulin were increased at 2 weeks in the SW9 group, and soluble CD8 and β2-microglobulin remained elevated at 3 months. Therefore, measles immunization resulted in suppression of lymphoproliferation, which was most evident in infants with the highest antibody responses and most immune activation.