Several lines of evidence suggest that islet-specific T cells are important in the pathogenesis of the insulitis resulting in insulin-dependent diabetes mellitus (IDDM). Therefore, we decided to analyze islet-specific T cell reactivity in the peripheral blood of IDDM patients. With the use of insulinoma membranes as antigen, T cell lines were generated from peripheral blood mononuclear cells of patients with recent onset of the disease. In a proliferation assay such T cell lines responded to insulinoma membranes and, though to a lesser extent, also to fibroblast membranes, the control antigen used. One of the T cell lines was cloned. Eight clones were isolated that respond to insulinoma antigens. Five of these eight clones appeared to be specific for insulinoma membranes, i.e. they demonstrated proliferation in response to insulinoma but not fibroblast membranes. These insulinoma-specific proliferative responses are HLA-DR restricted.