Rubella vaccine was administered to 289 preschool children in the winter and spring of 1969-1970. Among children with seronegative reactions, 94% acquired rubella hemagglutination-inhibition (HI) antibodies; 10 of the 13 who had remained negative responded with low titers to second doses of the same vaccine. Immunization histories were verified in over 85% of the children. Rubeola HI antibodies were present in 90% and those to mumps and vaccinia in 35% and 62%, respectively. Almost half had A2/HK/3/68 and A2/Jap/305/57 influenza antibodies. Poliovirus neutralizing antibodies were not detected to type 1 in 45% and to type 3 in 37%. The importance of maintaining continuing serological surveillance is emphasized by these data.