In 4- and 5-month-old infants in whom the undiluted chick embryo fibroblast (CEF) Schwarz strain measles vaccine had a poor immunogenic effect, there was an increase in immunogenicity when the high sugar concentration was diminished without reference to added albumin. The human diploid cell (HDC) measles vaccine was still superior in this age group even in a lower concentration of the Ikić, Edmonston-Zagreb strain of virus. More aerosolized, HDC Ikić strain virus was required for high seroconversion rates in 4- and 5-month-old infants who had higher titers of prevaccination plaque-neutralizing (PN) antibodies. Some of these infants had a delayed immune response that was absent at six weeks but present at three months after vaccination. The data provided evidence that the PN and enzyme-linked immunosorbent assay techniques measured different antibodies that develop and persist in different ways in 4- to 5-month-old infants. The HDC lyophilized measles vaccine yielded unexpectedly high seroconversion rates after subcutaneous injection of 5,000 plaque-forming units (PFUs) in 4-, 5-, and 6-month-old infants: 69% 89%, and 100%, respectively, at 14 weeks. In 12- to 23-month-old infants there was seroconversion of 92% and 100% at six weeks after inhalation of an estimated 175 PFUs of the CEF vaccine and 375 PFUs of the HDC vaccine, respectively. Within six weeks after vaccination, the PN antibody titers were significantly higher with the CEF vaccine (geometric mean titer of 2,275) than with the HDC vaccine (geometric mean titer of 343).