Beta-2-microglobulin (beta 2m) plasma levels during hemodialysis have recently been considered markers of membrane biocompatibility. The aim of this study was to assess if generation of beta 2m from blood cells in contact with the membrane could account for changes in beta 2m plasma concentration during hemodialysis. The role of heparin was studied as well by comparing conventional with heparin-free dialysis. beta 2m plasma levels were measured in six patients during 12 Cuprophan (Cu) and 12 Eval (E) non-consecutive hemodialysis sessions without ultrafiltration. E membranes were compared with and without heparin. beta 2m plasma levels significantly increased with Cu but not with E. A total of 200 ml of blood from four other patients was recirculated ex vivo before Cu hemodialysis in Cu or E and then reinfused into the patient. A drop in beta 2m concentration was observed in the recirculating blood, recovery of beta 2m from the washed filter was higher with E, and reinfusion of recirculated blood to the patient was not associated with significant increases in beta 2m plasma levels. Hemodialysis with a new Cu filter, following reinfusion, did not produce significant increase in plasma beta 2m concentration. In conclusion, 1) beta 2m plasma levels change during Cu, but are not associated with hemoconcentration from ultrafiltration nor with local release from the blood leaving the filter, 2) beta 2m binding to the membrane may account for decreased levels in recirculated blood, 3) E membranes both with and without heparin do not increase beta 2m plasma levels and reinfusion of recirculated blood blunts the beta 2m changes observed with Cu.