Using a specific immunoradiometric assay method the in vitro alpha interferon response to polyinosinic:polycytidylic acid (poly-I:C) and the gamma interferon response to concanavalin A were measured in peripheral blood mononuclear cells from 11 healthy matched pairs of Type 1 diabetic patients and normal subjects. The alpha-interferon response to poly-I:C was significantly higher in the diabetic group (median 3.7 (range less than 1-25.7) u ml-1) than in the normal group (1.1 (less than 1-15.4) u ml-1, p less than 0.01). The mean gamma-interferon response to concanavalin A was 64.3 +/- 46.9 (+/- SD) u ml-1 in the diabetic patients and 49.4 +/- 18.5 u ml-1 in the normal group (NS). The higher alpha-interferon response to poly-I:C in the diabetic patients was not related to blood glucose concentration, HbA1, age of onset of diabetes, duration of diabetes, or islet cell antibody positivity, and may therefore indicate intrinsic hyper-responsiveness of circulating mononuclear cells in Type 1 diabetes.