A cellular enzyme-linked immunosorbent assay (ELISA) was used to detect circulating anti-endothelial cell antibodies (AECA) in sera of patients suffering from rheumatoid arthritis (RA). Felty's syndrome (FS), systemic lupus erythematosus (SLE), progressive systemic sclerosis (PSS) and those with a "lupus anticoagulant" (LA). IgG AECA were detected in RA, FS, SLE and LA sera, while IgM AECA were only detected in RA and FS. AECA were not specific for endothelial cells. However, IgG binding to endothelial cells and dermal fibroblasts was F(ab) mediated while in all other cell types tested, nonspecific binding most likely via the Fc region occurred. In RA and FS rheumatoid factor was shown to augment immunoglobulin binding to endothelial cells. Additional studies revealed that some of these pathological sera also contained cytotoxic IgG autoantibodies which fixed complement and damaged human umbilical vein endothelial cells in vitro. These studies suggest that a group of autoantibodies, present in a variety of collagen vascular disorders, react with endothelial cells and thus may be important aetiopathogenic factors in the vasculopathies associated with these disorders.