A method is presented for evaluating the influence of porosity on the early reactions of blood with polymer membranes of cellulose acetate and polyethersulfone with two different pore sizes, 0.1-0.2 microm and 0.8 microm. A system of two phases consisting of capillary blood and Dulbecco's phosphate buffered saline was constructed. Platelet adhesion and exposure of thrombospondin was examined by immunofluorescence. Platelet adhesion was higher on the membrane with 0.8 microm pore size. Leukocyte adhesion and viability was measured by fluorescein diacetate/propidium iodide staining, and the respiratory burst response to PMA and opsonized zymosan was measured by chemiluminescence. Leukocyte viability was higher on the membranes with 0.8 microm pore size and higher on the cellulose membrane for exposures upto 2 h. After 3 h the leukocyte viability was highest on the polyethersulfone membrane with a pore size of 0.8 microm. The respiratory burst response of membrane-adhering leukocytes could not be triggered by opsonized zymosan on any of the tested membranes. A response was seen after stimulation with PMA of cells adhering to the cellulose membrane with a pore size of 0.8 microm.