The chronic tissue damage associated with long-term diabetes mellitus may arise in part from in situ immune complex formation by accumulated immunoglobulins and/or antigens bound to long-lived structural proteins that have undergone excessive nonenzymatic glycosylation. In this report, we have tested this hypothesis using nonenzymatically glycosylated collagen. Binding of both albumin and IgG averaged four times the amount bound to unmodified collagen. Both albumin and IgG (anti-BSA) bound to nonenzymatically glycosylated collagen retained their ability to form immune complexes in situ with free antibody and antigen.