We report on the use of magnetoelastic sensors for distinguishing the effects of fibrin networks, platelet aggregation, and thrombin generation in the clotting of whole blood. Samples of bovine whole blood are treated with reptilase, ADIP, and kaolin activators to generate clots based on fibrin networks, platelet aggregation, and thrombin respectively. The strengths of these clots are measured by tracking the resonance amplitude of the magnetoelastic sensors immersed in these blood samples as a function of time; the magnitude of the change in resonance amplitude, as the blood sample transforms from liquid to solid, increases with increasing clot strength. The characteristic behavior of the magnetoelastic sensors under different clotting conditions is used to quantify the extent of platelet aggregation due to ADP activator without interference from fibrin or thrombin activities. This work establishes the effectiveness of the magnetoelastic sensing technology as a simple, portable, and cost effective tool for platelet function analysis and quantifying the effects of anti-platelet therapies. Copyright © 2008 American Scientific Publishers. All rights reserved.