22 000 serum samples from university students and young adult blood donors collected between 1969 and 1980 were tested for antibody to rubella virus. Of the women born in 1956 and after (i.e., those eligible for rubella vaccine at school) the proportions seronegative ranged from 3% to 6% in different years, compared with 12-20% in men of the same age groups. Serum samples from schoolchildren were also tested: 40-50% of girls and boys aged 10-11 years were seronegative--percentages similar to those before the introduction of vaccine. However, only 2-6% of 15-year-old girls (who would have been offered vaccine) were seronegative compared with 20-30% of 15-year-old boys. These results indicate that the rubella vaccination programme in the U.K. is having a significant impact on the immunological status of young women of childbearing age but that a large number of women remain potentially susceptible.