Malignant mesothelioma (MM) is a pulmonary malignancy that appears to be immunogenic based on a large number of studies in both animals and humans. This notion is supported by our recent demonstration using Western blot analysis of immunoglobulin G antibodies reactive with a variety of autoantigens in many patients with MM. In view of the enormous potential of such antigens in early diagnosis, immunotherapy, and vaccination of at-risk individuals, it was essential to identify these antigens. We therefore applied the SEREX technique (serologic identification by recombinant expression cloning), using a serum pool from six patients as the probe against an expressed complementary DNA library derived from a cloned MM cell line. We screened over one million recombinants and obtained sequence information on eight antigens that had provoked immunoglobulin heavy chain class switching, presumably as a consequence of T-cell recognition. Six of these antigens were identifiable (U2AF[65], Siah binding protein, topoisomerase IIbeta, ZFM1, mIre1, and pendulin), and of the others, one was found as a single EST from a myotube library (Jemm-1); the other (Jemm-2) was not represented in any EST database even as a weak homolog. Consistent with our previous findings, each of the characterizable antigens would be expected to be associated with the cell nucleus. Each of the autoantibody specificities was uniquely associated with a single patient with the exception of antibodies to TOPIIbeta and U2AF(65). We found 13 of 14 (93%) patients with MM had antibodies to TOPIIbeta and two of 14 (14%) patients had antibodies to U2AF(65). The number of serum reactivities, taken as a measure of the complexity of the immune response, correlates with patient survival and with an index of systemic inflammation. These data suggest that a broader range of serologic reactivities reflects a more active host response to the presence of tumor.