Islet-cell antibodies have been reported to be of predictive value for the future development of Type 1 (insulin-dependent) diabetes in first degree relatives of diabetic patients with the risk increasing in these subjects with the islet-cell antibodies titre. However, very little is known about islet-cell antibodies in background populations. Sera (n = 8363) from schoolchildren (6-17 years) in the French background population were screened for the presence of islet-cell antibodies by the indirect immunofluorescence technique. Islet-cell antibodies greater than 4.5 Juvenile Diabetes Foundation units were found in 150 sera (prevalence rate 1.8%; 95% confidence interval 1.5-2.1%). Only 17 sera demonstrated islet-cell antibody titre greater than or equal to 24 JDF units. No particular feature was found to be significantly different between islet-cell antibody-positive and islet-cell antibody-negative children (age, family history of diabetes, fasting plasma glucose, insulin autoantibodies). A second blood sample was obtained from 80 of 150 islet-cell antibody positive children after a mean interval of 8 months. Only 11 sera became less than 4.5 JDF units with islet-cell antibody titres being stable in the remaining sera, including the high-titre positive sera (greater than or equal to 24 JDF units). HLA-DQB typing was performed by restriction mapping techniques in 80 islet-cell antibody-positive, in 93 islet-cell antibody-negative and in 213 Type 1 diabetic children. The distribution of the susceptibility alleles (DQB1-Asp57-negative) was not significantly different between islet-cell antibody-positive and islet-cell antibody-negative children.(ABSTRACT TRUNCATED AT 250 WORDS)