Seventy-two insulin-treated diabetic patients were classified on the basis of a clinical evaluation of their control of diabetes. There were 39 stable patients, 23 unstable patients and 10 patients of intermediate degree of control. Four insulin resistant patients were also studied. Serum insulin antibodies were measured in each patient and the concept of insulin buffering by its antibody was developed. Most unstable patients had low concentrations of insulin antibody. Twenty-four of the 39 stable patients had a significant concentration of insulin antibody and 15 patients had low levels of antibody. The insulin resistant patients had hugh levels of antibody. All unstable patients had low antibody buffering and all insulin resistant patients had high antibody buffering. Although many stable patients had buffering antibodies others lacking antibody required a low insulin dose and their stability of diabetic control was attributed to residual pancreatic function.