Minimodule hollow fibre dialysers, representing clinical dialysis modules on a scale of 1/25, enable quantitative evaluation of the haemocompatibility of hollow fibre membranes in an ex vivo flow system in humans. On line heparinization, adjusted for donor sensitivity, is maintained at a minimal level (approximately 0.14 units/ml). Blood samples collected at the minimodule exit over 30 min are analysed for heparin (anti-Xa activity), activated partial thromboplastin time, fibrinopeptide A, platelet count and beta-thromboglobulin, complement fragment C3a, leucocyte count and polymorphonuclear neutrophil elastase. Initial experiments were performed using well-characterized reference materials: acrylonitrile copolymer (AN 69 HF), polysulphone and cuprophan (CUP). Activation of coagulation and platelets was low for AN 69 HF, intermediate for CUP and greatest for polysulphone, while complement activation was negligible in the presence of AN 69 HF, moderate for polysulphone and most important for CUP. Future applications will be directed towards haemocompatibility screening of prototype membranes with the aim of developing clinical dialysers with improved biocompatibility.