A synthetic polycarbonate (PC) membrane supplied by C.R. Bard Inc. was assessed as to its clinical usefulness and suitability to regular use with artificial kidney. The permeability of the PC membrane to 11 solutes of increasing molecular volumes (Na+, Ca++, K+, Cl-, HPO4-, urea, creatinine, uric acid, glucose, BSP, cyanocobalamine) was measured in vitro by rotating dialysis cells, as compared to that of Cuprophan PT 150. The evaluation of the PC membrane in vivo was carried out during a regular hemodialytic treatment in 5 patients using a Kiil dialyzer. The dialysance of 6 solutes (HPO4-, urea, creatinine, uric acid, hypaque, cyanocobalamine) across the PC membrane was measured at 200 ml/min blood flow rate. Both in vitro and in vivo the PC membrane showed permeability and dialysance coefficients to small molecules approximately the same than standard PT 150; the ultrafiltration rate of the PC membrane was also superimposable to that of PT 150. On the contrary, larger molecules were removed much more efficiently by the polycarbonate membrane than by PT 150. These data suggest that the membrane evaluated in this study possesses some peculiar features which could possibly result in an improvement of the quality of regular hemodialysis.