In 14 patients beta 2-microglobulin serum concentration before and after haemodialysis using cuprophane capillary dialyzers with 0.7; 1.2 and 1.5 m2 surface was measured. beta 2-microglobulin concentration did not change during the haemodialysis procedure using 0.7 m2 dialyzers and was 31.15 +/- 7.58 mg/l before the dialysis and 31.10 +/- 13.59 mg/l after the procedure. Using 1.2 m2 dialyzers beta 2-microglobulin serum level increased (not significantly) from 29.40 +/- 7.53 mg/l before dialysis up to 36.29 +/- 11.70 mg/l after dialysis. When employed 1.5 m2 dialyzers the increase of beta 2-microglobulin serum concentration was higher and statistically significant (p < 0.02). The values of beta 2-microglobulin serum level before and after the haemodialysis were 29.89 +/- 2.44 mg/l and 38.04 +/- 5.89 mg/l respectively. There was a significant increase of number of patients with higher beta 2-microglobulin serum level (p < 0.01) according to the increase of dialyzers surface. beta 2-microglobulin concentration after the haemodialysis procedure using 0.7 m2 dialyzers was lower than calculation of protein changes could show. However using 1.2 and 1.5 m2 dialyzers beta 2-microglobulin serum level was markedly higher (statistically significant (p < 0.05) when employed 1.5 m2 dialyzers), than expected using the some above calculation. The increase of beta 2-microglobulin showed positive, but statistically not significant correlation with the index of haemodialysis intensitivity. The above mentioned data indicate that the increase of beta 2-microglobulin after haemodialysis is not related to biocompatibility of cuprophane membrane, but is dependent on intensivity of haemodialysis, which associated with the surface of the membrane.