We report here the identification and preliminary characterization of the messenger RNA coding for a Mr 50,000 plasminogen activator inhibitor (PAI) synthesized by cultured bovine aortic endothelial cells. Polyadenylated RNA was prepared from these cells and translated in a rabbit reticulocyte lysate in vitro translation system. When the 35S methionine labeled translation products were immunoprecipitated with monospecific antiserum to PAI and analyzed by SDS-PAGE and autoradiography, a single major polypeptide of Mr 40,000 was revealed. Competition experiments were performed to determine the relationship of the immunoprecipitated polypeptide to the PAI. The amount of 35S-labeled immunoprecipitate was greatly decreased by the presence of the purified PAI, consistent with the conclusion that the Mr 40,000 band represented the translation product of PAI mRNA. This mRNA migrated with a sedimentation coefficient of 22s when analyzed by sucrose gradient centrifugation. The in vitro translation assay was used to determine the relative amount of PAI mRNA in cells cultured in calf serum purchased from different vendors. The level of PAI mRNA varied by at least eight-fold depending on the serum employed, suggesting that expression of the PAI gene is subject to regulation by external factors.