The immune status of children vaccinated prior to 1 year of age was investigated during a measles epidemic in an industrial section of Connecticut. Of the 61 documented cases which occurred in former recipients of live vaccine 46% were children vaccinated prior to the age of 1; children vaccinated before age 1 comprised 1/3 of the total vaccinated population under study. The disproportionately increased number of cases in children vaccinated before their 1st birthday was not statistically significant. Of children vaccinated before age 1 80% had demonstrable measles antibody. The geometric mean antibody titer of those vaccinated in infancy was lower than that of children vaccinated after the age of 1 (p<0.01). Following revaccination 20 of 21 seronegative children who had been vaccinated before age 1 showed an IgG response without IgM; both IgM and IgG appeared following vaccination in 7 previously unvaccinated children. A child with measles encephalitis who had previously received live vaccine at 10 months of age also showed a secondary antibody response. The study suggests that infancy is only a relative contraindication of live measles virus vaccination; it further suggests that capacity for anamnestic response is not necessarily protective against measles. (authors)