The reactivity of islet cell cytoplasmic antibodies (ICA)-positive and ICA-negative sera of recent onset type 1 diabetic patients was studied in human fetal pancreata of 12-18 weeks' gestation and compared with the reactivity of these sera in adult human control pancreata. The aims of the study were: (1) to observe the presence of ICA staining in human fetal islet cells; (2) to compare endpoint titres (in Juvenile Diabetes Foundation units) of ICA-positive patient sera in fetal pancreata and adult human control pancreata. Ten ICA-positive sera and eight ICA-negative sera from newly diagnosed diabetic patients and four sera from healthy controls were tested on three human adult and eight human fetal pancreata. As in the adult control pancreata. ICA-positive sera reacted to insulin-, glucagon-, and somatostatin-positive cells of fetal pancreata of all gestational ages. This was observed both in single cells and in cells in islet-like cell clusters. Dilution of a reference serum gave similar results in both adult and fetal pancreata. In contrast, the ICA-positive patient sera yielded a striking heterogeneity in fetal as well as in adult pancreata. However, end-point titres between adult and fetal pancreata did not differ significantly (P>0.05). In conclusion, ICA-positive sera from recent onset diabetic patients show that the expression of molecules to which ICA react is present in all islet cells and starts before week 12 of gestation.