Microparticles are small membrane vesicles released from activated cells and are associated with thrombosis and inflammation. Microparticles contain a unique subset of surface proteins derived from the parent cell and may be responsible for their diverse biological functions. To identify these proteins, juvenile baboons (Papio anubis, n = 4) underwent iliac vein thrombosis with 6-hour balloon occlusion. Plasma samples were taken at baseline and at 2 days postthrombosis for microparticle analysis. Microparticles were extracted from platelet-poor plasma, digested separately with trypsin and tagged using isobaric tagging for relative and absolute quantitation reagents. The digests were subjected to 2-dimensional liquid chromatographic separation followed by matrix-assisted laser desorption/ ionization tandem mass spectrometry. Peak lists were generated and searched against all primate sequences. For protein identity, a minimum of 2 peptides at 95% confidence interval was required. Later, isobaric tagging for relative and absolute quantitation ratios were generated comparing relative protein level of day 2 to baseline. The proteomic analysis was performed twice for each blood sample, totaling 8 experiments. Proteins were considered elevated or depressed if the isobaric tagging for relative and absolute quantitation ratio deviated by 20% change from normal and a P value less than .05. Significantly, 7 proteins were differentially expressed on day 2 compared to baseline, and appeared in at least 3 animals and regulated in at least 4 experiments. Among these 7 proteins, upregulated proteins include various forms of fibrinogen and α-1-antichymotrypsin and downregulated proteins include immunoglobulins. These proteins influence thrombosis and inflammation through hemostatic plug formation (fibrinogen), inhibiting neutrophil adhesion (α-1-antichymoptrypsin), and immunoregulation (immunoglobulins). Further studies are needed to confirm the mechanistic role of these proteins in the pathogenesis of venous thrombosis.