Takayasu's arteritis or non-specific aortoarteritis is an inflammatory and stenotic disease of the aorta of questionable aetiology. Immunopathogenic mechanism, the precise nature of which is uncertain, is often suspected to be one of the basic causes of this disease. The present study was designed to estimate the antiaorta antibody titre in Takayasu's arteritis patients and to further locate the antigen in the vessel wall. Thirty clinically and angiographically proven cases of Takayasu's arteritis patients with appropriate controls were studied. Antiaorta antibody titres were estimated using Enzyme-Linked Immunosorbent Assay method. The controls included patients of vascular diseases other than Takayasu's arteritis, autoimmune diseases like rheumatoid arthritis and systemic lupus erythematosis and normal healthy individuals. Absorbance value at 492 nm at a dilution of 1:500 of the patients' sera was expressed as the antiaorta, antibody titre. There was significant difference (p < 0.005) between the mean value of the antibody titre in patients (0.471 +/- 0.073) and patients of other vascular diseases (0.209 +/- .056); autoimmune diseases (0.143 +/- .024); and, controls (0.108 +/- 0.012). Collagenase treatment of the aorta resulted in the fall of the antibody titre of aortitis patients (0.162 +/- 0.036) suggesting that the collagen might be one of the components responsible for autoantigenecity of aorta resulting in aortitis. The aortic extract was further subjected to 10 percent sodium dodecyle sulphate-polyacrylamide gel electrophoresis and immunoblot was done with Takayasu's arteritis patients' sera as well as controls' sera. The sera in 80 percent of Takayasu's arteritis patients immunoprecipitated a protein of molecular weight 45,000 (45 kilodalton) whereas only 15 percent patients of autoimmune disease group showed precipitation band though of lower molecular weight. Normal human sera gave no immunoprecipitation band. The precise nature of the antigen still needs to be identified.