Rubella virus (RV) infection of the fetus in the first trimester of pregnancy usually results in severe birth defects collectively termed Congenital Rubella Syndrome (CRS) and is frequently associated with prolonged RV persistence in the infant. Immunological tolerance to RV is believed to contribute to viral persistence, but the mechanism for this is unknown. In this study, RV-specific antibody responses in CRS patients and healthy controls who had experienced Rubella infection postnatally were compared to determine if there were differences that might account for RV persistence in the former group. Levels and functional affinities of IgG specific for individual RV proteins (E1, E2, and C) were measured by enzyme immunoassay (EIA). Relative amounts of RV protein-specific IgG directed to linear and topographic epitopes were compared by immunoblots run under reducing or nonreducing conditions, respectively, and biological activity was determined by hemagglutination inhibition (HAI) assay. Results showed that both CRS patients and control subjects had comparably high levels of IgG directed to whole RV and to RV E2 and C proteins as measured by EIA. However, in contrast to the controls, CRS patients were found to have significantly reduced levels of antibodies directed to RV E1 protein and its linear (but not topographic) epitopes. Also, functional affinities of specific IgG directed to whole RV and E1 protein, as well as hemagglutination inhibition titers, were found to be significantly lower in CRS patients than in controls. The data suggest that intrauterine exposure to RV may result in selective immunological tolerance to the RV E1 protein. A model is presented that accommodates the serological findings of this investigation within a proposed mechanism of RV persistence resulting from selective immunological tolerance to RV E1 protein.