By means of the method divised by the authors two types of capillary dialyzers were used many times: MTS C 1.0 model by Fresenius, membrane thickness 11 mu, and 23-08 model by Travenol, membrane thickness 8 mu. At the fourth use of the Fresenius dialyzers it was found in the studies performed at the start of haemolysis that urea clearanic decreased by 8% on the average. This, with the blood flow of 200 ml/min, was significant statistically (p less than 0.05). In the Travenol dialyzers the decrease was by 18% on the average, while the decrease that was statistically significant was when the dialyzer was used for the second time (p less than 0.05). Creatinine clearance at the fourth use of the Travenol dialyzers decreased by 20% on the average--insignificantly, and in the Fresenius dialyzers it decreased significantly at the second use (p less than 0.01), though at the fourth haemodialysis it decreased by 10% on the average. Clearance of urea and creatinine at the end of haemodialysis decreased more prominently than at the start. This caused an increase in the difference between the start and the end of the dialysis, which in the Travenol dialyzers was made visible as late as at the fourth use of dialyzer. The authors think that the data show the capillary dialyzers can be used three or four times.