In an attempt to define diagnostic criteria that may help to distinguish the congenital rubella syndrome (CRS) from subclinical intrauterine rubella virus (RV) infection, maternal and fetal serum samples were analyzed using (1) enzyme immunoassay employing RV synthetic peptides as antigen, (2) IgG avidity assay, and (3) immunoblot under nonreducing conditions, in addition to hemagglutination inhibition and commercial enzyme immunoassays. Infants born with CRS and their mothers were shown to reveal low or undetectable levels of E2-specific antibodies and deficient IgG recognizing the major neutralizing antibody-inducing epitope on the E1 protein (SP15). Antibody responses were normal in mothers with presumed RV reinfection as well as in asymptomatic infants born after maternal primary rubella. The results indicate that the maturation of specific humoral immune responses is obviously less efficient when intrauterine RV infection results in CRS. The detection of high avidity IgG, conformational E2-specific as well as SP15-reactive antibodies may serve as a potential predictor for a benign outcome of intrauterine RV infections.