A new modified cellulose dialysis membrane (MC, 1.2 m2) in which less than 5% of cellulosic OH-groups are substituted by tertiary amino groups was tested in a 6-week clinical trial for biocompatibility and clinical performance and compared to both regenerated cellulose (RC, 1.2 m2) and cellulose acetate (CA, 1.0 m2). Ten patients on maintenance hemodialysis took part in the study; all hollow fiber modules were equally well tolerated and no adverse reactions were observed. Using MC/RC/CA, mean clearances after 15 min of hemodialysis (HD) amounted to 162/169/150 ml/min for urea, to 143/143/124 ml/min for creatinine, and to 104/107/84 ml/min for phosphate (QB = 200 ml/min, QD = 500 ml/min, UFR = 0). Mean drop of systolic blood pressure was 6/10/8 mmHg and mean decrease of heart rate averaged 3/3/3 beats per minute, respectively, during the first hour of HD as compared to starting conditions. Residual blood volume in the modules after HD was low and heparin consumption identical for all 3 membranes (34 IU/kg X h). Mean peak C3a generation for MC/RC/CA amounted to 1312/3486/3099 ng/ml, respectively; leucocyte and platelet counts dropped to a minimum of 67/24/47% and 81/86/91%, respectively during the first hour of HD as compared to initial values. Elastase release from PMNL and platelet factor 4 from platelets showed no significant differences between the membranes. In conclusion, the new MC membrane showed a significantly better biocompatibility and equally good clinical performance as compared to RC and CA.