During dialysis with cuprophane or polycarbonate filters, a fall in neutrophil and eosinophil cell counts is observed. Total complement remains unchanged, but C3a increases indicating complement activation. Release of granular granulocyte proteins is observed indicating granulocyte activation. Patient reactions or biochemical findings indicate no significant difference when polycarbonate or cuprophane membranes are used, except perhaps less complement activation with polycarbonate. The biochemical observations may be due to complement activation or cell contact with the membrane.