Insulin- and anti-immunoglobulin-antibodies have been recently reported in pre-diabetic subjects: the former has been proposed as a predictive marker of Type I diabetes in non-diabetic-subjects. To evaluate the diabetes-related specificity of these antibodies, the presence of insulin autoantibodies, using a recently developed and highly sensitive competitive radioimmune assay, and of anti-immunoglobulin antibodies together with that of immune complexes and of other autoantibodies has been investigated in patients with organ- or non-organ-specific autoimmune diseases. One hundred and eleven serum samples were assayed from patients with Graves' disease, primary hypothyroidism, chronic autoimmune thyroiditis, Addison's disease, chronic autoimmune hepatitis, pernicious anemia, lupus erythematosus, and rheumatoid arthritis, together with 45 serum samples from normal subjects. From patients with autoimmune diseases, 32.4% of all sera revealed values of insulin autoantibodies above the limit of positivity (p less than 0.001); anti-immunoglobulin antibodies were present in 4.1% of patients (NS); immune complexes were found in 19.5% (NS) of all patients, but in 38% of patients with Graves' disease and chronic hepatitis (p less than 0.02). There was a trend for multiple autoantibody positivity to be associated with high levels of insulin autoantibodies (p less than 0.05). Thus, whereas contrary to expectation anti-immunoglobulin antibodies are not associated with non-diabetes-related autoimmune diseases, increased humoral immunoresponsiveness to endogenous insulin appears to be related to autoimmunity in general rather than restricted to Type I diabetes.