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Nearly one-third of IgM antilymphocyte autoantibody-positive sera from patients with systemic lupus erythematosus (SLE) contain IgM antibodies to one or more 180-220-kD molecules (p180, p190, p205, and p220) in blots of glycoproteins purified from T cells by wheat germ agglutinin affinity chromatography. Identity of these IgM targets with multiple isoforms of CD45 was established by their specific depletion from T cell glycoproteins by immunoprecipitation with T191, a monoclonal antibody (mAb) that reacts with an epitope common to all CD45 isoforms. Although the anti-CD45 autoantibodies recognize higher molecular weight isoforms primarily, antigenic specificity in this system is quite heterogeneous and includes multiple distinct CD45 isoforms on different types of T cells that are, at least in part, different from those reactive with mAbs 2H4 and UCHL-1. Because CD45 is a major membrane protein tyrosine phosphatase that plays a critical role in antigen- induced T cell activation, the present data may be relevant to some of the antilymphocyte antibody-mediated immunologic abnormalities that characterize SLE and related autoimmune diseases.