The effect of blood-membrane interaction on several biocompatibility parameters has been investigated. Four groups of healthy subjects underwent sham hemodialysis, i.e. the establishment of blood-dialysis membrane contact, but without circulating dialysate, using cellulose-based membranes (regenerated cellulose and cellulose acetate) or synthetic membranes (polysulfone and polyacrilonitrile). Contact between blood and cellulose-based membranes resulted in pronounced complement activation and leukopenia whereas contact between blood and synthetic membranes induced weak complement reaction. The release of granulocytic elastase comparable to that obtained during clinical dialysis seemed to correlate directly with the types of membrane used. The gradual increase in the level of plasma elastase during blood-membrane contact, as opposed to the transient nature of the increase in the levels of complement, suggests that different mechanisms are responsible for these reactions. Nutritional implications of dialysis membrane bioincompatibility are discussed in the light of recently published metabolic data obtained in these subjects.