Significance During initial growth of a clot, it is necessary for platelets to aggregate, which is mediated by binding fibrinogen molecules that bridge platelets through their integrin receptors. How platelet integrin receptors rapidly bind fibrinogen that is attached to the surface of another platelet but yet ignore soluble fibrinogen remains a mystery. This is difficult to understand since fibrinogen is the third most abundant protein in blood plasma. Here we show that differentiation between soluble and immobilized platelet ligands is mediated by mechanical forces. We demonstrate that the platelet integrins apply specific piconewton forces to test their ligands within the platelet junction. Our results provide insights into how clotting functions.