Under the auspices of WHO an investigation was made by 9 laboratories in different parts of the world on the distribution of rubella antibodies in girls and women of child-bearing age. In the first part of the study the objective was to determine the reliability and reproducibility of the tests employed. It was found that there were no significant differences in the variability of the titres obtained in different laboratories when the results were compared with those obtained by repeatedly testing the same sera in one laboratory.In the second part of the study sera were obtained from girls in schools and women attending clinics and health centres. They were not taken from random samples of the populations. In most of the studies the pattern of development of antibody was similar. About half the persons had antibody at 6-8 years of age and 80%-87% at 17-22 years of age, the percentage remaining relatively constant thereafter. The island populations of Trinidad and Jamaica and a rural area of Japan were, however, found to have significantly fewer women with antibodies than urban areas in Europe or the Americas.