BK-PMMA 2.1 m2 surface dialysers were used in vivo and in vitro for evaluation of their capacity for beta 2-M removal by both adsorption and elimination through the membrane. As expected, in vivo, beta 2-M plasma concentrations were more effectively reduced when the dialyser was used in haemofiltration, rather than in haemodialysis. However, the mass recovered in the ultrafiltrate could not account for the observed difference in beta 2-M removal. In vitro tests demonstrated that membrane adsorption of beta 2-M is greater when the dialyser is used in convection than in diffusion. Therefore, the greater beta 2-M removal during haemofiltration depends on both increased adsorption and membrane elimination by convection. Thanks to adsorption, beta 2-M removal by haemodialysis was greater than for other synthetic membranes, but was insufficient to obtain beta 2-M removal corresponding to the estimated generation.