The autoimmune aetiology of type I diabetes has been well documented. We studied whether anti-insulin anaphylactic antibodies were present on the membrane of basophils from type I diabetics by the toluidine blue method (detecting basophil activation after stimulation by insulin). We observed that basophils of recently diagnosed insulin-dependent diabetic patients (n = 13) were statistically more frequently activated by insulin than basophils from noninsulin-dependent diabetics (p < 0.002, n = 8) or non-diabetic subjects (p < 0.05, n = 9). Basophils from normal donors were passively sensitized with plasma from insulin-dependent diabetics and could then be activated by insulin. This sensitization still occurred when using plasma previously heated to 56 degrees C, indicating that the sensitizing antibodies were not of the IgE class. When basophils from type I diabetics were preincubated with anti-IgG subclasses, only anti-IgG4 monoclonal antibodies inhibited the insulin-induced basophil activation. By contrast, preincubation with blocking concentrations of anti-IgG1-3 antibodies or desensitization of the IgE pathway did not modify basophil activation. These experiments strongly suggest the presence of anti-insulin antibodies of the IgG4 subclass in insulin-dependent diabetics before any insulin administration and provide a simple tool to complement the usual method of detecting auto-antibodies in diabetes.