Serum from a patient with the CREST Syndrome and systemic lupus erythematosus contained an IgM antibody that reacted at dilutions up to 1:800 with a fibrous cytoplasmic network in several epithelioid and fibroblastic cell lines. The antibody was shown by immunofluorescence microscopy to label a specific subset of cytoskeletal polymers, the intermediate filaments. The reactive antigen from this biochemically heterogeneous group of filaments was established as the 58,000-mol wt protein, vimentin: (a) the patient's serum reacts with a range of cell lines that contain intermediate filaments composed of vimentin, but not with cells whose intermediate filaments are composed of different protein subunits; (b) in PTK2 epithelioid cells the serum reacts with the class of filaments that coils around the nucleus after colchicine treatment (vimentin) and not with the filaments that remain dispersed after colchicine (prekeratin); and (c) the component of reactive cells that combines with the serum is shown by immunoelectrophoresis to be a 58,000-mol wt protein antigen. A similar antibody that binds intermediate filaments of PTK2 cells was encountered at lower titer in some sera from other patients with connective tissue diseases and in control sera. Previous routine antinuclear antibody assays using mouse liver or commercially prepared HEp-2 cells have failed to reveal anticytoskeletal antibodies in patient sera, perhaps due to inadequate presentation or preservation of cytoplasmic antigens.