Complement activation on hydroxyl-group-bearing surfaces is regarded as the main reason for granulocyte activation in applications of blood-contacting medical devices such as extracorporeal blood purification. However, the factors inducing the cell adhesion so far remained ambiguous. For a dedicated research, whole blood was incubated with a set of structurally similar polymer coatings on glass with either hydroxy or ether functionalities. By co-incubation of an activating with a non-activating surface, the reaction of granulocytes activated by complement fragments on non-activating surfaces could be evaluated. As expected, hydroxyl-terminated polymer layers induced much higher levels of complement activation than those with ether functionalities. Leukocyte activation, as measured by the expression of CD11b, correlated closely with the presence of free complement fragment C5a. However, adhesion of leukocytes was rather associated with the adsorption of activated fragments of C3 than with the activation level of the cells. Moreover, it was found that adsorbed quantities of fibrin and fibrinogen had little influence on leukocyte adhesion. It is concluded that the activation of leukocytes is triggered by soluble complement factors such as C5a while their adhesion on hydroxy-bearing surfaces is mainly triggered by the presence of surface-bound complement fragment C3b.