Sera from rats inoculated with in vitro glucosylated rat skin acid-soluble collagen were evaluated for specific antibody response using the enzyme-linked immunosorbent assay. The data indicate that the altered protein is capable of eliciting the production of antibodies, but unmodified collagen is not. The specificity of the generated antibodies appears to be directed toward the glucitollysine residues of the modified collagen. Sera from streptozotocin-induced diabetic rats contain antibodies that clearly bind glucosylated collagen. These results provide support for the hypothesis that in diabetes mellitus increased collagen glucosylation during chronic hyperglycemia may be the initiating event of an autoimmune response leading to the long-term complications.