In the present study we prepared by chemical modification a series of porcine pancreatic phospholipase A, (PLA) derivatives, that bind to the activated glycoprotein (GP) IIb/IIIa complex and hydrolyse phospholipids in the outer leaflet of the platelet membrane. To the native enzyme, an RGD-containing peptide was coupled to introduce affinity for GPIIb/IIIa in combination with lauric acid to improve binding to the membrane. As controls, derivatives containing only one of these modifications were prepared. Acylation of the enzyme improved the affinity for densely packed phospholipids, as deduced by kinetic analyses. After stimulation of platelets, the RGD-containing PLAs bound to GPIIblIlIa since GRGDS peptide and a monoclonal antibody against the complex interferred with binding. No binding was found with native PLA. The binding seen with lauric acid PLA was not mediated by GPIIb/IIIa. All modified PLAs induced 1 -3 % hydrolysis of ['Hlarachidonic-acid-labelled phospholipids in resting platelets. After activation with a-thrombin, hydrolysis increased to 17 %, corresponding to about 90% of ['Hlarachidonate-labelled phospholipids in the outer leaflet of the plasma membrane. RGD-containing PLAs were more active than lauroyl PLA, and their activity was mediated via GPIIb/IIIa since GRGDS inhibited release of ['Hlarachidonic acid. Acylation of the RGD-containing PLAs did not further improve the hydrolytic properties. We conclude that chemical modification of PLA leads to a targetted hydrolytic action and could be a basis for the design of enzymes that specifically destroy activated platelets.