A double-blind, placebo-controlled comparison of single component and combination measles, mumps, and rubella (HPV-77:DE-5 and RA 27/3) virus vaccines involving 502 young children was conducted. The rubella antibody response was similar with RA 27/3 rubella and measles-mumps-rubella (RA 27/3) vaccines, but was diminished with the combination vaccine that incorporated HPV-77:DE-5 rubella. There was no evidence of enhanced clinical reactivity with either of the measles-mumps-rubella vaccines.