The pronounced leukopenia caused by cuprophane dialyzer membranes was significantly blunted by citrate regional anticoagulation. Cellulose acetate produced less leukopenia than the cuprophane, regardless of anticoagulant. The pO2 response to the initiation of hemodialysis was not affected by dialyzer membrane or anticoagulant choice. We therefore conclude that citrate anticoagulation reduces dialyzer-induced leukopenia. Citrate anticoagulation does not, however, change the hypoxemia present with acetate dialysis. The dissociation of leukopenia and hypoxemia with citrate anticoagulation suggests that pulmonary sequestration is not a major cause of hypoxemia during hemodialysis.