Rubella antibody (IgG) has been measured in females reporting for antenatal screening using single radial haemolysis (SRH) and time-resolved fluorescence immunoassay (TRFIA). We have shown TRFIA to be a simple, specific, highly sensitive, quantitative assay for rubella IgG with a lower limit of detection of 0.2IU/ml. Out of 506 sera tested by SRH, 18 (3.6%) had low levels of antibody (< 15IU/ml) compared to 83 (16.4%) tested by TRFIA and, of these, 32 (6.3%) had rubella antibody concentrations < 10IU/ml. The lowest level (3.1%) of rubella susceptibility (antibody levels < 10IU/ml) was found in females aged 25-29 and the highest level of susceptibility (23.5%) occurred in females aged 40 years, and over. Geometric mean rubella antibody concentrations (IU/ml) were 26.8, 34.4, 34.8, 29.7, 27.5 and 20.0 for age groups <20, 20-24, 25-29, 30-34, 35-39 and > or =40 years, respectively. Our rubella vaccination policies have built up good levels of rubella immunity in women of childbearing age in our locality, and using TRFIA technology we can accurately monitor changes over time.