We studied the expression of Mac-1 on granulocytes (flow cytometry) from 9 patients (27 examinations) during hemodialysis with cuprophane membranes (CuM). Samples were drawn before dialysis and from the arterial and venous lines at 5,15 and 180 min. Granulocytopenia was most pronounced after 15 min of treatment. Mac-1 expression on granulocytes increased during dialysis, and a pronounced enhancement occurred across the dialyzer. Mac-1 expression on granulocytes increased 116% across the dialyzer at 5 min, and the degree of Mac-1 mobilization at 5 min correlated with the degree of subsequent granulocytopenia at 15 min. In in vitro experiments, granulocytes from healthy blood donors were incubated with plasma drawn from the arterial and venous lines at 5 min. Plasma drawn from the venous line at 5 min had a greater ability to mobilize Mac-1 than plasma from the arterial line (p < 0.002). This difference correlated with the degree of granulocytopenia at 15 min (r = 0.76, p = 0.01). Pieces of a washed CuM were incubated in normal human serum (NHS) preparations for different times. Inactivation of the alternative pathway of complement activation (NHS-50°) did not alter the generation of Mac-1-mobilizating capacity compared to NHS. In contrast, inactivation of the classical pathway (NHS-EGTA) decreased the early but not the late generation of Mac-1-mobilizating capacity. Our results indicate that the early mobilization and not the absolute expression of Mac-1 correlates with granulocytopenia, and that sera in vitro attain Mac-1-mobilizating capacity more rapidly when the classical pathway of complement activation is intact