We studied the upregulation of the intracellular glycoprotein Mac-1 (CD11b/CD18, CR3) on monocytes and granulocytes during 36 bicarbonate hemodialyses in 12 patients who were randomly treated with Cuprophan (Cu), Hemophan (He) or Polysulfone (PS; low-flux) membranes. The degree of mobilization of this adhesion protein was related to changes in granulocyte and monocyte count, generation of C3a and production of interleukin-1 beta in plasma. Mac-1 expression on granulocytes was significantly higher after 5 and 15 min of Cu hemodialysis as compared to He or PS dialyses (p < 0.001) and correlated to changes in granulocyte count at 15 min (r = 0.62 and r = 0.76, p < 0.001). No differences in early Mac-1 mobilization on circulating monocytes was observed despite a decrease in cell count. Mac-1 expression on monocytes and granulocytes in the venous blood line at 180 min of treatment was significantly higher during Cu dialysis as compared to He and PS dialyses (p < 0.02 and p < 0.001, respectively). Early generation of C3a was higher in patients on Cu dialysis than in He or PS dialysis (p < 0.001) and correlated both to granulocytopenia (r = 0.45, p < 0.01) and to the subsequent increase in Mac-1 expression on granulocytes (r = 0.63, p < 0.001). An early increase in Mac-1 expression on monocytes was accompanied by an increase in plasma interleukin-1 beta later during dialysis (p < 0.05). Studies of Mac-1 expression during hemodialysis increased the sensitivity of biocompatibility measurements and correlated better than complement generation to changes in granulocyte count as it mediates adhesion to endothelial cells.