The influence of substrate wettability on leukocyte adhesion was studied using a series of polyurethane films with different surface wettabilities, prepared by a two step gas plasma modification procedure. In the first step the films were made hydrophobic by exposure to a tetrafluoromethane plasma. In the second step hydrophilicity was gradually increased by exposure to an argon plasma. XPS analysis of these films demonstrated that the first step resulted in the incorporation of fluor into the film surface (50 At.-%). In the second step fluor was almost completely removed from the surface, while the surface oxygen concentration increased from 5 to 20 At.-%. The wettability of the modified films, as characterized by water contact angle measurements, increased as a function of the argon treatment time. The advancing/receding contact angles (Wilhelmy plate method ) decreased from 128/83 to 50/2 degrees respectively. In vitro adhesion of granulocytes and lymphocytes to the modified surfaces was studied under static conditions as a function of time. For both types of leukocytes it was found that the number of cells which adhered to the surface within a certain incubation time increased when the surface became more hydrophilic. The number of adherent granulocytes was about 2-3 times higher than the number of adherent lymphocytes. The number of adherent cells which could be predicted from the relationship between leukocyte adhesion and the wettability of the substrates within the series of homologous surfaces corresponded with the number of adherent cells to control surfaces (untreated polyurethane and Teflon).