Patients of some rheumatic autoimmune diseases like systemic lupus erythematosus have in their sera autoantibodies to vimentin, the main protein of intermediate sized filaments of the cytoskeleton of connective tissue cells. Immunoprecipitation and Western blotting techniques have been used to detect the interaction between human anti-vimentin autoantibodies and the two peptides that resulted from the cleavage of vimentin by N-chlorosuccinimide. Both methods and also immunoabsorption of sera with the isolated peptides suggest that the interaction of vimentin and immunoglobulins is mainly carried out through a peptide of 30kD mol. wt located close to the amino-terminal part of the vimentin molecule. The same results were obtained when a serum from a rabbit immunized with human vimentin was used.