The objective of this study was to develop proteomic profiles that would distinguish between resting and activated platelets in a clinical setting using surface-enhanced laser desorption/ionization (SELDI) time of flight (TOF) technology. A data set of 50 donors was analyzed. Distinct spectral patterns emerged in the low-molecular-weight range (2-10 kDa) for resting platelets and platelets aggregated with adenosine diphosphate (ADP) or thrombin receptor activation peptide SFLLRN (TRAP) and in platelets exposed to shear stress. Platelets from patients treated with ADP receptor antagonists did not show the expected change in proteomic profile following aggregation with ADP. These data provide the first demonstration that proteomic signatures of platelets can be developed using SELDI-TOF in a clinical laboratory setting.