Islet cell antibodies (ICAs) in Chinese (23 IDDM, 13 NIDDM and 6 non-diabetic) were characterized for immunoglobulin isotypes and light chain specificity. All ICAs were IgG-type and none were IgM-or IgA-type (median titre: 20 JDF units; range 10-160). Light chain specificity showed that 25/36 (69.4%) of the diabetic patients had lambda and kappa chains. Half of the non-diabetic subjects had both lambda and kappa chains. The rest had only lambda chains. Isotyping for ICA-IgG subclass combination with IUIS/WHO reference monoclonal antibodies in the diabetic patients gave the following: IgG1, alone - 9 (25%), IgG1+2+3- 8 (22.2%), IgG1+2- 11 (30.6%), IgG1+3- 6 (16.7%), IgG2+3-2 (5.6%). No ICA-IgG4was detected. The frequency of the subclasses would be: IgG1, - 94.4%, IgG2- 58.3% and IgG3- 44.4%. The distribution of ICA-IgG subclasses was not affected by diabetes type (IDDM or NIDDM) or duration of disease. Of the 6 non-diabetic subjects only one had a single ICA-IgG subclass (IgG1,). Serum levels of IgG subclasses in a subgroup of the patients (n = 16) were not significantly different from normal individuals. Biochemical modification of pancreatic tissue prior to ICA testing showed that acetylneuraminic acid residues, lipid and protein components were associated with binding of ICAs. The co-existence of other autoantibodies was also tested in these 42 ICA-positive sera. Twelve individuals (1 non-diabetic) had thyroid autoantibodies. Antibodies to thyrotrophin receptor, gastric parietal cell and rheumatoid factor were not detected. These results indicate that ICAs in Chinese are not associated with IgG1subclass restriction, but are polyclonal in nature and require acetylneuraminic acid residues, lipid and protein components for binding.