To define the concentration of anti-rubella virus (RV) antibodies discriminating nonimmune from immune persons and to characterize immune responses to rubella vaccination, serologic studies were performed after rubella vaccination in persons with low or undetectable antibody concentrations. Thirty-six subjects with primary immune responses had prevaccination anti-RV IgG concentrations <15 IU/mL by ELISA and negative results by radial hemolysis. Eighty-three subjects with secondary immune responses had mean IgG increases of 9 IU/mL within 2 weeks. Eight of them had initial IgG levels <15 IU/mL, and 2 were negative by radial hemolysis. Both groups attained similar antibody levels after 1-3 months. Secondary immune responses to rubella vaccination were delayed by >2 weeks and thus resembled the time course of primary immunization, but IgM responses and IgG avidity were distinct between subjects with primary or secondary immune responses. Thresholds for immunity <15 IU/mL entail the risk of withholding rubella vaccination from susceptible persons.