This work describes an in vitro system developed to quantitate platelet deposition on different dialysis membranes. The system is based on the use of small dialysis filters and reproduces the haemodynamic pattern of blood flowing through hollow fibres during in vivo dialysis. We have determined the in vitro platelet adhesion to cuprophan and to a non-cellulosic membrane, polymethylmethacrylate. When albumin concentration in the platelet suspension was low (0.35%) platelet deposition to cuprophan and to polymethylmethacrylate was comparable. When albumin concentration was increased to a physiological value (3.5%) platelet adhesion to both cuprophan and to polymethylmethacrylate membranes significantly decreased. This effect of albumin was greatest for the high-permeable polymethylmethacrylate membrane (BK). These data suggest that platelet membrane interaction is significantly influenced by circulating albumin.