This review describes the physiology and biochemistry of phagocytic cells and examines the rationale for using their function to assess biocompatibility. Haemodialysis modulates phagocyte adhesion receptors to a degree dependent on the level of complement activation by the membrane, and this modulation leads to a proportional change in the number of circulating phagocytes because of pulmonary sequestration of activated cells. However, it is not clear that other acute changes in phagocyte function are directly attributable to the nature of the membrane; they may merely reflect changes in the population of circulating cells resulting from pulmonary sequestration. While haemodialysis is associated with stimulation of monocyte cytokine production, the relative contributions to this phenomenon of the membrane material, its mass transfer properties, and the microbiological quality of the dialysate are incompletely understood. A lack of well-designed studies and difficulty in separating the effects of uraemia from those of dialysis make it impossible to know if chronic changes in phagocyte function are a consequence of the choice of dialysis membrane. Thus, while changes in the number of circulating phagocytes and their expression of adhesion receptors are useful indices of biocompatibility, a clear association has yet to be shown between membrane properties and changes in other measures of phagocytes function.