The layer-by-layer assembly between cationic chitosan and anionic dextran sulfate was analyzed quantitatively by a quartz crystal microbalance technique in the absence and presence of 0,2. 0.5. and 1 M NaCl in the polymer solution. The apparent film thickness increased upon increasing the NaCl concentration. The anti-versus procoagulant activity of these films against whole human blood was Studied by (lie immersion of a substrate into blood for 30 min incubation time at 37 degreesC. The substrate was coated with films of varying NaCl concentrations and assembly step numbers. There was a critical concentration for the alternating activity; above a concentration of 0.5 M NaCl. both anti- and procoagulation could be observed on the dextran sulfate and chitosan surfaces. respectively. The underlying layer of the assembly was necessary for this alternating activity after a five-step assembly. the activity was realized, The adsorption of a cationic dye (methylene blue) onto the films revealed that the anionic-charge density derived from dextran sulfate on the film surface was linearly increased with increased NaCl concentration. There was a critical charge density of the dextran sulfate for the anticoagulant activity. An assembly was also constructed from I combination of chitosan and heparin, but the activity was different from that of the former system strong anticoagulant activity was observed even on the chitosan surface. We suggest that the polymer species and/or the assembly conditions are key factors for realizing the alternating bioactivities of films prepared by the layer-by-layer assembly.