Rapid occlusion of artificial microvascular grafts (AMGs; less than or equal to 2-mm diameter) by platelet-rich thrombi prevents the clinical use of AMGs in cardiac, vascular, and plastic surgery. Since present antiplatelet agents are unable to assure AMG patency, we have studied the role of specific platelet membrane-glycoprotein blockade on platelet retention by AMGs. In a customized in vitro perfusion chamber, retention on polytetrafluoroethylene (PTFE) AMGs (1.0-mm i.d.) of indium-111-labeled platelets in human whole blood was measured in the presence and absence of inhibitors. Specific blockade of platelet membrane glycoprotein (Gp) IIb/IIIa was achieved using monoclonal antibody 10E5 (10 micrograms/ml) and the peptide GRGDS (Gly-Arg-Gly-Asp-Ser, 0.75 mM). These inhibited 98% and 35%, respectively, of platelet retention under circumstances in which aspirin (7 mM) and dextran (4 mg/ml) inhibited 19% and 18%, respectively, of platelet retention. Nonspecific immunoglobulin G F(ab')2 (10 micrograms/ml) and nonspecific peptide (GGDA; Gly-Gly-Asp-Ala, 0.75 mM), used as control reagents, were ineffective in this setting. Monoclonal antibody 6D1 (10 micrograms/ml), which blocks platelet membrane GpIb, prevented 82% of platelet retention on PTFE. These doses of 10E5 and GRGDS completely inhibited platelet aggregation in response to 20 microM ADP, and the dose of 6D1 completely inhibited ristocetin-induced platelet agglutination. The aspirin dose prevented the second phase of ADP-induced aggregation. These data indicate that not only does initial platelet adhesion to PTFE require GpIIb/IIIa but also that GpIb plays a significant role in the early stages of platelet retention on PTFE AMGs.