The interactions of two insoluble anticoagulant polystyrene derivatives with human platelets were studied in an in vitro system similar to a chromatography column. Blood was pumped through the column and platelets were counted before and after passage through the column. Interaction of platelets with the beads led to retention of platelets in the column. The same experiment was performed with several donors. Different pretreatments were assayed and compared for both materials: platelet poor plasma, antithrombin III-depleted platelet poor plasma and an antithrombin III solution. Platelet retention depends on the polymer composition: the material containing glutamic acid sulphamide groups, which has a larger anticoagulant activity in plasma than the materials only substituted by sulphonate groups, is always less reactive towards platelets. The differences in the effects of pretreatment on both materials can be correlated with the variations of antithrombin adsorption on the synthetic surfaces.