['To establish monoclonal antibodies (mAb) against megakaryocyte potentiation factor (MPF) and detect MPF in the blood of patients with mesothelioma.', 'Mice were immunized with a purified recombinant human MPF-rabbit-Fc fusion protein and with MPF. Several hybridomas producing mAbs to MPF were established. A double-determinant (sandwich) ELISA was constructed using mAbs to two different epitopes and used to determine if MPF is present in the serum of patients with mesothelioma.', 'We established seven anti-MPF mAbs whose topographical epitopes were classified into three nonoverlapping groups. All the mAbs reacted with recombinant MPF protein by ELISA. One of the mAbs detected MPF and the mesothelin precursor protein containing MPF in cell lysates on Western blotting. A sandwich ELISA using mAbs to two different epitopes was constructed and used to measure the presence of MPF in the media of various mesothelin-expressing cancer cell lines and in human serum. The ELISA showed that MPF levels were elevated in 91% (51 of 56) of patients with mesothelioma compared with healthy controls. Furthermore, serum MPF fell to normal levels in two patients after surgery for their peritoneal mesothelioma.'] Is megakaryocyte potentiation factor cleaved from mesothelin precursor a useful tumor marker in the serum of patients with mesothelioma?