A study was carried out in the Cordillera Health District (Santa Cruz Department, Bolivia) from October 1988 to April 1989 to determine the seroprevalence of measles antibodies and the seroconversion rates among a group of previously unvaccinated children (9-36 months of age) from the urban and rural area of the province, before and after immunization with a standard dose of Schwarz measles vaccine. Among 265 previously unvaccinated children, 77 (29%) had measles IgG antibodies prior to immunization; 141 out of 147 (96%) seronegative children at the time of vaccination seroconverted. No difference in seroprevalence and in seroconversion rates was found between the urban and rural groups.