(1) During the first ten years after primary vaccination with live measles virus vaccine, a slow decline in the level of HAI antibody titres has been demonstrated. The decline after more attenuated vaccines was at the same gradient, but at lower titres.(2) During the same period after primary vaccination by inactivated (; killed ') followed by live - K + L - measles vaccine, a much steeper decline in the level of HAI antibody titres was demonstrated.(3) The GMT may not reach the lowest standard titres for over 40 years after the least attenuated vaccine, and 20-25 years after more attenuated vaccines. After K + L vaccine the GMT may be negative after 15 years or less, but this will depend on the frequency of natural boosting.(4) On revaccination after five to ten years, one out of 20 children (five per cent) who had received live vaccine alone for primary vaccination showed a fourfold or greater rise in titre of HAI antibody, whereas 17 out of 41 children (41%) who had received K + L vaccine showed a significant rise in titre.(5) With either regime, the age at primary vaccination did not appear to influence the rate of boosting after revaccination except perhaps in children under two years.(6) Among those who originally received K + L Vaccine, The Lower The Hai Antibody Titre Before Revaccination And The Longer The Time Since Primary Vaccination, The More Likely Was A Significant Rise In Titre To Occur After Revaccination. All Should Be Revaccinated Within 10-15 Years Of Primary Immunisation.(7) Children whose primary vaccination was by live ;further attenuated' vaccine before the age of two years may need to be revaccinated before they leave school, but this requires further study.(8) A case can be made for using a less attenuated strain of measles virus for revaccination than for primary vaccination.(9) The eventual need for and timing of revaccination against measles should be examined in a larger group of children.