Thrombin stimulation alters the membrane surface of platelets so that specific components on the membrane surface interact. To identify such “aggregation factors”, tne exposed membrane proteins of washed platelets were labeled by lactoperoxidase-catalyzed iodination and tested for their association with cytoskeletal structures. Control, thrombin-stimulated (TS; nM thrombin in mM EDTA to prevent aggregation) and thrombin aggregated (TA; 2 mM Ca++) platelets were treated with 1% Triton X-100. The insoluble material (isolated by centrifugation) from TS platelets, but not unstimulated platelets, had clusters of filamentous material with dense cores about 1 μ in diameter. Each cluster appeared to arise from one platelet and contained proteins with the Mr of actin actin-binding protein and myosin plus a 56K and 90K protein. Triton extraction of TA platelets produced an insoluble material with a similar protein composition as that from TS platelets; however, the filamentous clusters remained- aggregration, indicating tnat membrane components which aggregate platelets were still present. Analysis of iodinated membrane components revealed that all were solubilized by Triton from control and TS platelets while two glycoproteins, termed IIb and III, remained with the filamentous material from TA platelets. This and the observation that platelets lacking IIb and III cannot aggregate [JCI. 60: 535 (1977)], indicate that one or both of these membrane glycoproteins are involved in the direct Interaction of platelets during aggregation.