Takayasu's arteritis is an inflammatory arteritis of unknown origin. It affects the aorta, its main branches and at times the pulmonary artery. Takayasu's arteritis is a worldwide disease, however, there is predilection to affect young women of mongoloid ancestry, therefore most cases do occur in the Far East and Latin America. For more than 50 years a relationship with mycobacterial infection has been sought without definite proof. In this investigation we found a circulating IgG antibody in the sera of Takayasu's arteritis patients, which recognizes a 38 kD glycoprotein of M. tuberculosis. This glycoprotein is an specific antigen in the human immune response against mycobacterial infection.