The aim of the study was to compare under in vivo conditions the new polyamide (Polyflux 160, Gambro) and cuprammonium-rayon dialyzers (AM-FP-17, Asahi; Clirans SE15NL, Terumo) with the polysulfone dialyzer (F60, Fresenius) regarding their permeability for beta-2-microglobulin (11.8 kD), retinol binding protein (21 kD), alpha-1-microglobulin (26.7 kD), alpha-1-glycoprotein (41 kD), alpha-1-antitrypsin (54 kD), albumin (66.3 kD) and transferrin (90 kD). The marker substances were measured by nephelometry or radioimmunoassay. To evaluate the membrane permeability, sieving coefficients 20 min after the start of hemodialysis were calculated using the concentration in the afferent and efferent blood line and in the ultrafiltrate. To get an idea about convective protein loss during a hemofiltration session, the values were computed in milligrams per 20 liters ultrafiltrate. Concerning the permeability of beta-2-microglobulin, the F60, AM-FP-17 and the Polyflux 160 hemofilter were comparable. In the molecular weight range of 20-60 kD both synthetic hemofilters were nearly impermeable. However, the cuprammonium-rayon dialyzers showed in this range a higher cutoff. The calculated albumin and transferrin loss is lower than those of CAPD patients or patients with nephrotic syndrome.