We recently reported that the serum from a patient with lupus nephritis, insulin resistance, and hypoglycemia contains multiple populations of antibodies directed at the human insulin receptor. In the present study, we found a subpopulation of antibodies (eluted from a protein A-Sepharose affinity column at pH 4.3) directed at the human fibroblast insulin receptor. When tested against human placental membranes, IM-9 lymphocytes, circulating monocytes and erythrocytes,and isolated adipocytes, the antibody subpopulation did not compete with 125I-insulin for binding to its receptor. In contrast, the antibody subpopulation competed with 125I-insulin for binding to the human fibroblast insulin receptor. This antibody subpopulation stimulated [3H]aaminoisobutyric acid ([3H]AIB) uptake to these cells. Unlike the effect of insulin, however, this regulation of transport was not antagonizedby a mouse monoclonal antibody to the human insulin receptor that inhibits 125Iinsulinbinding. These studies indicate, therefore, that a tissue-specific antibody subpopulation can occur spontaneously in patients with antibodies to the human insulin receptor. Furthermore, they indicate the presence of anti-insulin receptor autoantibodies specifically directed against a tissue that is not primarily involved in glucose metabolism.