A complex array of inflammatory mediators are generated as a consequence of blood contact with hemodialysis (HD) membranes. Beside complement activation, other mediators are involved in cell activation, and are thought possibly to be responsible for early and long-term multiple changes in immunity infection, hypercatabolism, beta 2-microglobulin generation and hemostatic mechanisms. Previous studies from our laboratories have established platelet activating factor (PAF) as one of the mediators generated by complement-dependent or independent mechanisms of cell interaction with hemodialysis membranes. Recent studies on the production of PAF from endotoxin-primed polymorphonuclear neutrophils in a closed miniaturized circuit, and on the effect of PAF in mediating endotoxin- and cytokine-induced leukocyte adherence to HD membranes, highlight so far undescribed new roles of this mediator in biocompatibility.