The total rubella IgG antibody avidity ELISA, as described by Hedmann for differentiation between acute primary rubella and reinfection, persisting or non-specific IgM antibodies were evaluated in 212 sera of four categories of patients with known history (n = 149) and in one group (n = 39) of patients with a history of unclear nature. The development of the IgG ratio (avidity) from low (less than 30%) to higher levels (greater than 30%) was determined in 146 sera from 96 cases with acute primary rubella between days 1 to 256 after onset of exanthem, with a borderline between low (less than 30%) and high (greater than 30%) avidity after four weeks after onset of exanthem. From seven days to 14 years following vaccination, the IgG ratio increased more slowly following vaccination, with a useful borderline at 2 1/2 months. In 80 sera from 13 women with reinfection during pregnancy, six infants with rubella embryopathy, and 19 pregnant women with rubella IgM antibodies detected during antenatal screening, the total IgG ratio ranged between 48% and 100%, indicating a previous infection. The diagnostic value of the IgG subclass-specific antibody avidity assay was investigated in 40 of the 251 sera. The results confirmed those of the total IgG avidity assay. Both tests are of additional help for differentiating serologically primary rubella from reinfection or recent from previous vaccination.