The present study was designed to evaluate the influence of two haemodialysis membranes of different biocompatibility on red blood cell function. Twelve patients were studied in two consecutive dialyses, with cuprophan and cellulose acetate. Blood was extracted at 0, 20 and 180 min after the beginning of the haemodialysis session and general haematological parameters, osmotic fragility, deformability, methaemoglobin concentration and malonyldialdehyde (MDA) red blood cell content were determined. Osmotic fragility improved with both membranes, but this improvement was more marked with cellulose acetate. MDA red blood cell content showed a tendency to increase after 3 h with cuprophan (114 +/- 11% of the basal value), whereas it tended to decrease with cellulose acetate (92 +/- 12%), the differences between the two groups being statistically significant. These results suggest that red cell function may improve by changing the characteristics of haemodialysis membranes. This phenomenon could be related to a better biocompatibility.