Searching for a dialyser membrane with a cut-off similar to that of the human glomerulus, a modified cuprammonium rayon (AM-75-UP) and a polyacrylonitrile (PAN-15-DX) haemofilter were tested in vivo for the ability to eliminate substances of a molecular weight (MW) of 10-65 kilodaltons (kDa). Endogenous marker substances of a defined MW (beta-2-microglobulin 11.8 kDa; retinol binding protein 21 kDa; alpha-1-microglobulin 26.7 kDa; alpha-1-glycoprotein 41 kDa; alpha-1-antitrypsin 54 kDa; albumin 66.3 kDa) were measured by laser nephelometry or immunosorbent assay; sieving coefficients (SC) and protein elimination (PE) per 20 l haemofiltration were calculated for each low-MW protein. The PAN haemofilter shows elimination characteristics comparable to those of earlier tested haemofilters (polysulphone, AN69, cellulose triacetate) with a sharp cut-off in the MW range of 10-15 kDa. The cuprammonium rayon haemofilter is permeable for proteins with a higher MW; especially for alpha-1-microglobulin a relevant removal (SC 0.2; PE 0.56 g/20 l) was established. This membrane has a cut-off more similar to that of the human glomerulus; but far from the demanded quality with a relevant removal of substances in the MW range up to 60 kDa. Calculated albumin loss (2.4 +/- 0.2 g/20 l) seems to be tolerable for stable haemodialysis patients.