Measles serum antibody levels were determined by plaque reduction neutralization (PRN) test in 1,075 children in the age bracket of 5 to 17 years who received a single dose of measles-mumps-rubella (MMR II) vaccine at one year of age. Of these, 297 children (28%) had measles PRN titres < 120 which may not be protective against measles infection. The proportion of susceptible children by age ranged from 14 to 35%; however, there was no consistent age-dependent trend in susceptibility rates. The study data indicate the decline in protective immunity occurs before five years of age, and the proportion susceptible increases only slightly thereafter. This supports the current move towards a two-dose immunization strategy in the control and elimination of measles, with the second dose being given before school entry. The present data also underscore the need to consider a mass catch-up immunization program in the interim to prevent potential outbreaks of measles in school settings. The combination of the above approaches, if implemented as soon as possible, can potentially eliminate indigenous measles in Canada by the year 2000, the target date set by the Pan American Health Organization.