We report the use of magnetoelastic sensors for studying platelet aggregation in fresh bovine blood and platelet rich plasma. The resonance amplitude of gravimetric magnetoelastic sensors immersed in blood/plasma samples mixed with EDTA, ADP, and Collagen are measured as a function of time. The sensors immersed in ADP and Collagen activated blood show a greater change of resonance amplitude compared to the sensor in EDTA mixed blood. Such a response of the magnetoleastic sensors in ADP and Collagen activated blood is attributed to a higher mass loading effect on the sensor surfaces caused by settling of the aggregated platelets. Platelet aggregation in ADP activated blood is confirmed by platelet count measurements carried out with a standard hematology analyzer programmed for cow blood. The sensors detect aggregation of platelets in platelet rich plasma, but with a smaller change of amplitude attributed to reduced mass loading. This work establishes the utility of the magnetoelastic sensing technology as a simple, portable, and cost effective tool for platelet aggregation studies of whole blood as well as platelet rich plasma. Copyright © 2008 American Scientific Publishers. All rights reserved.