A nonobese patient with maturity-onset diabetes mellitus was hospitalized for treatment of an ulcer on his right foot. During this episode, his diabetic control worsened, and he proved unresponsive to exogenous insulin. Unresponsiveness to insulin persisted after healing of the ulcer. Tolbutamide therapy was then begun and produced a marked reduction in blood sugar levels. Withdrawal of the drug was accompanied by a progressive rise in blood sugar level. Intravenous infusion of regular pork insulin at rates of 45 U/h and single-component pork insulin at rates of 120 U/h had minimal effect on the blood sugar level. High levels of antibody to beef insulin were measured, with lower levels of pork insulin antibodies. C-peptide values were in a normal range before tolbutamide treatment and increased after use of the drug.