Nanoparticles are used more and more extensively in today's society, especially in the industry sector. Humans get exposed to nanoparticles daily but the effect is a topic that has not been fully explored yet and its effect on humans is still unknown.The purpose of this project was to investigate whether the size of nanoparticles is a factor that influences their effect on humans, mainly the effect on blood activation. In order to study this, nanopaticles of polystyrene with three different sizes (75, 120 and 260 nm) were selected and incubated in a human whole blood model, the Chandler loop. The samples from the Chandler loop experiments were analysed with three different ELISA's: C3a, terminal complement complex (TCC, sC5b-9) and thrombin-antithrombincomplexes (TAT).The results in this study indicate that the smallest nanoparticle has a higher potential for activating the coagulation system than the larger ones. The complement system did not seem to be significantly activated from the nanoparticles. More experiments needs to be done in order to get a better statistic value but just as it is the results look promising and there is a tendency for a higher activation of the coagulation system with the 75 nm nanoparticles.