One hundred and thirty-seven infants, 6 to 12 months of age, were given live rubella vaccine to more clearly define the efficacy of rubella immunization under 1 year of age and to correlate passively acquired rubella antibody with response to rubella vaccine. By testing serial blood specimens taken from ten infants from birth to 6 months of age, we defined the half-life of rubella HI antibody as 30 days; our data supported the observation that rubella HI antibody is rarely detectable after 6 months of age. In this study population, no rubella HI antibody was detectable in any study participant at the time of vaccine inoculation. However, only 69% of the infants under 10 months of age seroconverted after rubella vaccine inoculation, whereas over 90% of those 10 months of age and older responded positively. No correlation between failure to seroconvert and levels of passively acquired rubella antibody at birth was found. Results of this study suggest that other age related factors are of major importance in influencing the immunologic response of the infant to live rubella vaccine.