It is well known that surface coatings for medical devices can be made antimicrobial through introduction of silver nanoparticles. By virtue of their extremely large surface-to-volume ratio, the silver particles serve as a depot for sustained release of silver ions, despite the fact that silver is not readily oxidized. Antimicrobial coatings are especially important in connection with indwelling catheters with a high risk of bacterial line infections, such as central venous catheters (CVCs). This study specifically addressed the question what the impact of silver nanoparticles (exposed at the coating's surface) and/or the release of silver ions would be on coagulation of contacting blood. Studies, performed in vitro with fresh platelet-rich blood plasma (PRP) from 5 different healthy volunteer donors, clearly pointed out that: (i) the presence of silver nanoparticles correlates with accelerated thrombin formation upon contact of the coating with PRP; (ii) platelet activation is stronger as a result from the contact with silver nanoparticle-containing coatings as compared to other coatings which are devoid of silver. A series of titration experiments, in which the potential effect of silver ions is mimicked, revealed that the observed activation of blood platelets can be best explained through a collision mechanism. The results suggest that platelets that collide with silver, exposed at the surface, become activated without adhering to the surface. These new results point, rather unexpectedly, at a double effect of the silver nanoparticles in the coating: a strong antimicrobial effect occurs simultaneously with acceleration of the coagulation of contacting blood. This new information is, evidently, most relevant for the development of improved surface coatings for indwelling catheters (such as CVCs) which should combine antimicrobial features and close-to-zero thrombogenicity.