The rubella antibody titres were evaluated in 158 girls who had been seronegative prior to immunization and had been immunised 4 years previously (1971) with the rubella vaccine HPV77DE5. In 138 girls (87%) the 1975 titres were unchanged in comparison with 1971. Only in 5 girls (3.2%) the titres had decreased by up to 2 log2 steps. In two patients the titre reached the critical value of 1:8 which must be considered negative due to the high sensitivity of the haemagglutination inhibition test. A titre increase by two or more steps was observed in 15 girls (9.5%). A third of the titre increases might be due to reinfection with rubella wild virus which would correspond to a reinfection rate of less than 1% per year. Present knowledge indicates that reinfection during pregnancy does not endanger the unborn child.