Interaction of human plasmin with a monolayer culture of mini-pig aortic endothelial cells was studied by using the 125I-labelled enzyme. The binding of plasmin was time- and concentration-dependent. Equilibrium between bound and free enzyme was obtained within 90s, and Scatchard analysis indicated a high- and a low-affinity population of binding sites of approx. 1.24 X 10(4) sites/cell having a Kd of 1.4 X 10(-9) M and 7.2 X 10(4) sites/cell with a Kd of 2 X 10(-8) M respectively. Plasmin, bound to cell, was spontaneously released within 2 min, suggesting a rapid equilibrium. Chemical modification of the enzyme with phenylmethanesulphonyl fluoride or pyridoxal 5'-phosphate revealed that neither the active centre nor the heparin-binding site of plasmin was involved in the interaction with the endothelial cell. In terms of endothelial-cell receptors, the binding sites of cells for plasmin and thrombin were different: the two enzymes did not compete with each other, and the pretreatment of cells with neuraminidase or chondroitin ABC lyase resulted in a 50% decrease of thrombin or plasmin binding respectively. Arachidonic acid incorporated into phospholipids of the cell was released by plasmin, but a change in the rate of prostacyclin formation was not measurable. The interaction of plasmin with endothelial cells seems to be specific in the fibrinolytic system, since plasminogen did not bind to these cells under similar conditions.