HLA typing was performed on 232 rubella seronegative (HAI antibody titer <8) children immunized with RA 27/3 vaccine. High convalescentphase geometric mean antibody titers were noted in children one to five years of age, in girls one to 12 years of age, in subjects with AB blood type and in vaccinees with HLA types B14 and BW22. Of nine children with rubella titers≧512, 44 percent had HLA type A28; in contrast, only eight percent of the total group had A28 HLA type. HLA-A2 was present in four, and HLA types B12 and BW17 were present in three of the six children in whom seroconversion did not occur. Fifteen of 37 sibling pairs had identical antibody responses; in contrast, only 18 percent of randomly paired children had titers identical to each other. Vaccine-related symptomatology could not be associated with any HLA type. The results of this study suggest that infection with and antibody response to RA 27/3 rubella vaccine is influenced by genetic factors, which include cellular membrane determinants.