Cultured bovine aortic endothelial cells and human serum contain plasminogen activator inhibitors (PAIs) that are immunologically related. In the present study, the electrophoretic mobilities, molecular weights (mol wt), and activities of these PAIs were compared. When fractionated by agarose zone electrophoresis, both PAIs migrated with beta mobility as compared with the mobilities of human plasma/serum proteins. Two-dimensional electrophoretic analysis, employing agarose zone electrophoresis in the first dimension and sodium dodecyl sulfate-polyacrylamide gel electrophoresis in the second dimension, indicated that these beta-PAIs comigrated, both having a mol wt of approximately 50,000. The activity of the PAI in endothelial cell-conditioned medium is enhanced severalfold by treatment with either sodium dodecyl sulfate or guanidine. In preliminary experiments, we were unable to stimulate the PAI activity of undiluted serum by similar treatments. However, the PAI activities in both diluted serum and gel-filtered or electrophoretically fractionated serum were enhanced by treatment with these denaturants. The gel filtration studies also revealed that serum contains multiple forms of the beta-PAI. These forms may represent polymeric PAI and/or complexes between the PAI and other serum components. These findings indicate that the primary PAIs in bovine endothelial cells and human serum are not only immunologically related but are also biochemically similar.