Indirect immunofluorescence techniques employing sheep monospecific antisera to human IgG subclasses on unfixed cryostat sections have revealed the IgG subclass distribution in autoantibodies to pancreatic islets (ICA), thyroid epithelial (TMA), gastric parietal (PCA) and adrenal fasciculata (AdA) cells. Whereas antibodies were detected in all four subclasses in 21 of 27 TMA positive sera (mean IgG titre 29 +/- 5 . 2), 13 of 15 PCA (mean IgG titre 41 +/- 3 . 8) and eight of 14 AdA sera (mean IgG titre 10 +/- 3 . 1), only four of 35 ICA positive sera (mean IgG titre 45 +/- 3 . 6) reacted in all four subclasses. Approximately 50% of ICA positive sera showed a restricted polyclonal response to the 'common' pancreatic antigen and 12% of these sera reacted only with IgG2 subclass. The restriction rarely applied to co-existent thyrogastric antibodies in these sera and was independent of the ability of ICA to fix complement. Lesser subclass restrictions were observed in antibody responses to the 'common' antigen of the adrenal cortex.