Measles antibody seroprevalence was compared in Innu, Inuit, and Caucasian peoples of northern Newfoundland, Canada, who were immunized with a single dose of M-M-R-II (Merck Research Laboratories) vaccine. Healthy, volunteer schoolchildren (n = 606) were enrolled. Measles antibody was measured with a whole virus measles-specific IgG EIA. Native (Innu and Inuit) schoolchildren (n = 253) had a significantly higher seropositive rate (83%) after a single dose of measles vaccine compared to Caucasian (n = 353) children (76%; p = 0.025), and higher mean antibody levels after immunization compared to Caucasian children (1.74 EIA units, vs. 1.63; p = 0.06). Caucasian children were more likely to have been immunized after age 15 months (20.6% vs. 9.6%; p = 0.001). There was no significant difference in the mean time interval between immunization and blood sampling for natives versus Caucasian (8.0 years vs. 7.95 years; p = 0.49). After adjustment for time from immunization and age at immunization, there remained a marginally significant racial difference in seropositivity (OR = 1.65, 95% CI 0.96, 2.83, p = 0.068). The unadjusted odds ratio for seropositivity (comparing natives vs. non-natives, combining negative and equivocal results) was 1.66 (95% CI 1.06-2.59, p = 0.018). The higher measles-seropositive rate found among native compared to non-native Canadian children suggests that genetic and/or environmental factor(s) affect circulating antibody levels following immunization. The determination of these sources of variability may lead to the development of more efficacious vaccines or delivery strategies.