The solute removal characteristics and haemocompatibility of low-flux dialysers containing Cuprophan, cellulose acetate, polymethylmethacrylate (PMMA), and polycarbonate-polyether (Gambrane) membranes were compared in a multicentre cross-over clinical trial. While all four dialysers provided comparable removal of urea and creatinine, the dialyser containing PMMA membrane showed a reduced ability to remove phosphate compared to that containing Cuprophan membrane. Significant beta 2-microglobulin removal was obtained with the dialyser containing Gambrane membrane, whereas the other three dialysers had no impact on plasma beta 2-microglobulin concentrations. The ability to activate complement, measured as changes in the plasma concentrations of C3a des Arg and the terminal complement complex, and to produce leukopenia was greater for the dialyser containing Cuprophan membrane than for the other three. The ability to activate complement and cause leukopenia was not consistent among the remaining three dialysers and the degree of leukopenia could not be predicted from the level of complement activation. Neutrophil degranulation, as indicated by the release of elastase-alpha 1-proteinase inhibitor, occurred to a greater extent with the dialysers containing Cuprophan and Gambrane membranes. None of the dialysers was overtly thrombogenic as judged by changes in platelet count and plasma concentrations of the thrombin-antithrombin III complex. Our results demonstrate that although there are many similarities between dialysers containing low-flux membranes, there are also significant differences. These differences may enable improvements in therapy, while allowing continued use of low-flux dialysers.