We conducted a community-based survey in Santa Cruz city, Bolivia, to determine the age-specific prevalence of measles antibodies, determine factors associated with absence of detectable measles antibodies, and to compare results of salivary and serum measles immunoglobulin G (IgG) antibody assays. Serum samples from 1654 children were assayed for measles IgG antibody using the haemagglutination inhibition (HI) assay, and salivary samples were also obtained from 187 children and tested for measles IgG antibody using an antibody capture radioimmunoassay. Reported measles vaccine coverage in children aged 12-35 months was 77% (95% confidence interval [CI], 72-81%). Eighty-seven percent (95% CI 85-89%) had detectable HI antibody, but a high proportion had antibody levels below 200 miu (30-40% of 2-14 years old children). Measles seronegativity was associated with not being vaccinated against measles, a negative history of measles disease, living in the inner city, being a lifetime resident of Santa Cruz, and young age. Of 212 children without detectable measles antibody, 58% had a positive history of vaccination or measles disease, so that historical information was not sufficiently reliable to identify susceptibles. The salivary measles antibody assay was not sufficiently sensitive to be used for population screening; only 54% of 171 salivary samples from children who had detectable serum HI antibody were positive. A mass measles vaccination campaign of all children under 15 years of age is planned in Bolivia in 1994. Although only 7% of school-age children in Santa Cruz were seronegative, the effectiveness of a mass campaign in this age group depends in part on the response to revaccination of children with low, but detectable, antibody levels.