The effect of aggregation and secretion on membrane proteins was studied in washed human platelets. Reversible aggregation without secretion was stimulated by ADP and secretion without aggregation was stimulated by thrombin in the presence of EDTA. No loss of platelet surface glycoproteins occurred during reversible ADP-induced platelet aggregation, as measured by quantitative polyacrylamide gel electrophoresis analysis of platelets that were labeled with (125)I-diazotized diiodosulfanilic acid (DD(125)ISA) before ADP stimulation. Also, no new proteins became exposed on the platelet surface after ADP aggregation, as determined by DD(125)ISA labeling after stimulation. Thrombin-induced platelet secretion also caused no loss of platelet surface glycoproteins. However, after platelet secretion two new proteins were labeled by DD(125)ISA: (a) actin and (b) the 149,000-mol wt glycoprotein (termed GP-G), which is contained in platelet granules and secreted in response to thrombin. The identity of DD(125)ISA-labeled actin was confirmed by four criteria: (a) comigration with actin in three different sodium dodecyl sulfate-polyacrylamide gel electrophoresis systems, (b) elution from a particulate fraction in low ionic strength buffer, (c) co-migration with actin in isoelectric focusing, and (d) binding to DNase I. The identity of actin and its appearance on the platelet surface after thrombin-induced secretion was also demonstrated by the greater binding of an anti-actin antibody to thrombin-treated platelets, measured with (125)I-staphylococcal protein A.Therefore, major platelet membrane changes occur after secretion but not after reversible aggregation. The platelet surface changes occurring with secretion may be important in the formation of irreversible platelet aggregates and in the final retraction of the blood clot.