Besides the known factors contributing to the pathogenesis of diabetic nephropathy, the role of immune mechanisms in types I and II diabetes is discussed of late; the contribution of autoimmune mechanisms to pathogenesis of noninsulin-dependent diabetes (NIDDM) is virtually unknown. Seventy-six patients with NIDDM and 48 with insulin-dependent condition were examined. Under study were levels of antibodies to FxIA and renal glomerular basal membrane antigens in the blood sera of donors and patients with types I and II diabetes, as well as concentrations, size, and pathogenicity of immune complexes. Antibodies to FxIA antigen were detected in patients with both types of diabetes with diabetic nephropathy. Detection of circulating antibodies to FxIA antigen in more than 70% of diabetics in the absence of protein in the urine may be used as a test system for the laboratory diagnosis of diabetic nephropathy prestage and as a criterion for prescription as early as at the initial stages of nephroprotective agents.