An immunohistochemical study was undertaken in an attempt to localise immunoglobulin in sections of human advanced atherosclerosis with thinning of the media (sub-clinical periaortitis) and without thinning of the media as well as sections of artery from patients with clinical periaortitis. The findings were that in routinely processed sections of advanced atherosclerosis showing medial attenuation and in sections from cases of clinical periaortitis IgG, and to a lesser extent IgM, was localised to insoluble lipid, ceroid, within the atheroma itself. It is suggested that these observations support the hypothesis that chronic periaortitis has an auto-allergic cause and that the allergen may be a component of ceroid, which is elaborated within the atheroma.