Four types of high-flux hemodialyzers, Primus 2000 (high-flux polysulfone 2.0 m2), Altra-Flux 170 G (cellulose diacetate 1.7 m2), FLX-15 GW (polyester-polymer alloy 1.5 m2) and PAN-85 DX (polyacrylonitrile 1.7 m2) were evaluated in vivo. A total of 12 stable chronic hemodialysis patients participated in the study and each type of dialyzer was tested once in 9 of them. Blood samples for the measurement of BUN, creatinine, phosphate, uric acid, albumin and beta2-microglobulin (beta2M) were drawn before and 5 min after the end of the study dialysis. During dialysis, which was performed in all patients with a blood flow rate of 250 ml/min for 240 min, the dialysate (550-600 ml/min) was collected every hour and samples were drawn for the measurements of all the above substances. The mean total amount of low-molecular substances removed per session by each dialyzer was very close to 19.5 g for urea, 2.0 g for creatinine, 0.9 g for phosphate and 1 g for uric acid. The one-third (30-33%) of the above amounts were removed during the first hour of dialysis. Dialyzers' clearances for creatinine and uric acid were significantly higher in Primus dialyzer comparing to FLX-15 GW (p < 0.05) while the clearance for urea showed a borderline significance (p = 0.055). No difference was found either among Altra-Flux 170 G, FLX-15 GW and PAN-85 DX or between Primus and PAN-85 DX dialyzers. Phosphate clearance did not show any difference among the four dialyzers. The lowest amount of albumin removed per session was 0.75 g by PAN-85 DX and the highest 1.8 g by FLX-15 GW, while the equivalents for beta2M were 80 mg by Altra-Flux 170 G and 142 mg by PAN-85 DX. A significant adsorption of beta2M on these dialysis membranes was indicated by the combination of a satisfactory serum beta2M reduction ratio (post-/predialysis values = 0.52, 0.77, 0.60, 0.55) with a reduced beta2M clearance (23.9, 13.6, 20.2, 25.1 ml/min). During the first hour of dialysis, in comparison to the following time, the highest amounts of albumin and beta2M (expressed as percentage of total) were removed by the Primus 2000 dialyzer. Our results indicate that under conventional conditions small differences in the surface area of the high-flux dialyzers are unimportant regarding the removal of low molecules. However, the composition of the membrane seems to play an important role in the removal of high-molecular substances.