The increasing interest in biodegradable nanoparticles containing biomaterials such as poly(D,L-lactide-co-glycolide) (PLGA) and chitosan for drug delivery raises issues regarding the blood compatibility of these nanoparticles, since some nanoparticles, including carbon nanoparticles, can affect human platelet aggregation and cause vascular thrombosis. Therefore, the aim of this work was to investigate the effect of polymeric nanoparticles on human platelet function by measuring aggregation and receptor expression in vitro.PLGA, chitosan-PLGA and a series of chitosan nanoparticles were prepared by the single emulsion technique and ionotropic gelation method. The effects of these nanoparticles (0.01-100 microg/ml) on resting platelets, as well as on platelet aggregation and expression of receptors (GPIIb/IIIa and P-selectin) induced by agonists in platelet-rich plasma were examined using light aggregometry and flow cytometry.All tested nanoparticles at concentrations below 10 microg/ml did not modify platelet aggregation, showing that they may be used for the delivery of active molecules to the bloodstream.