We saw a 57-yr-old female non-insulin dependent diabetes mellitus (NIDDM) patient with antiinsulin antibody, in whom insulin-like growth factor-I (IGF-I) was shown to be effective as a blood glucose-lowering agent. Due to the presence of the antibody, the patient suffered from postprandial hyperglycemia and frequent hypoglycemia, which were uncontrollable even by multiple insulin injection therapy. In contrast to insulin injection (0.24 IU/kg body wt) which showed a delayed glucose-lowering effect, subcutaneous injection of recombinant human IGF-I (0.05-0.1 mg/kg body wt) caused a quick fall in plasma glucose levels. Prolongation of the glucose-lowering effect of "rapid-acting" insulin frequently caused subsequent hypoglycemia in this patient, but the effect of IGF-I seemed to have disappeared within the first three hours.