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Circulating IgG antibodies reactive with smooth muscle antigens have been found in 21% of patients with intrinsic asthma contrasting with an incidence of 2.9% in extrinsic asthma and 3.7% in chronic bronchitis. This antibody is neither organ nor species specific and does not cross-react with cardiac or skeletal muscle. In some sera the pattern of fluorescence is distinct from that commonly seen in patients with liver disease and has a `streaky' appearance outlining the surface of individual muscle fibres, similar to the inter-myofibrillary pattern of some cardiac antibodies. Other autoantibodies were found in seven of the eleven positive sera, suggesting a heightened immunological reactivity in these patients with intrinsic asthma. There is no evidence from this study that the presence of smooth muscle anti-bodies is related to pathogenesis of asthma. [ABSTRACT FROM AUTHOR]