The prevalence of measles antibody in Japan was surveyed with a newly developed gelatin particle agglutination (PA) test, and the results compared with those of the hemagglutination inhibition (HI) test. The two age-distribution curves of the PA antibody-positive rates at > or = 1:8 and > or = 1:32 were almost the same in all the age groups, except the less-than-1-year-old group for which the rate at > or = 1:8 was higher than that at > or = 1:32 (p < 0.05, chi 2 test). In the vaccinated children, all groups older-than-1-year of age had antibody-positive levels of 96% or more. In contrast, in the unvaccinated children, there was a sharp increase in antibody-positive rates between the 1- and 4-year-old groups, indicative that about 80% of the children were infected by wild measles virus at these ages. A significant number of PA antibody-positive specimens were antibody-negative (< 1:8) by HI. The percentage of specimens in this category, PA (+) but HI (-), was greatest in infants less than one year old, and least in young children, but it increased with age to 97% of the HI (-) specimens from adults of more than 20 years of age. The PA test therefore detected some measles antibodies that HI could not. This test is simple and useful for making serosurveys in both developed and developing countries.