The optimal number of times a hollow-fibre cuprammonium rayon (Terumo TAF) dialyzer could be used was determined in twelve patients. Each dialyzer was reused six times. There was a highly significant decrease in neutrophil count (46.9 +/- 6.9%, mean +/- s.e.m.), platelet count (9.6 +/- 3.8%, mean +/- s.e.m.) and serum C3 concentration (10.25 +/- 2.4%, mean +/- s.e.m.) 30 minutes after the commencement of dialysis The intensity of these changes decreased with the re-use of the dialyzers up to the fourth time. There was a small, but significant increase in serum beta 2-microglobulin concentrations after a 6-hour dialysis. The difference between pre- and post-dialysis serum beta 2-microglobulin concentrations appeared to be greater during the 5th and the 6th use (18.8% and 20.5% vs 5.0% during the first use). The clearance rates of urea and creatinine showed a trend to decrease from the fourth use onwards and the ultrafiltration coefficient was significantly reduced from the third use (8.5 +/- 4.0 mean +/- s.e.m.) onwards so that at the sixth use it was only 75.8 +/- 4.8%, (mean +/- s.e.m.) of the value at the first use. It is concluded that the optimal number of uses a cuprammonium rayon hollow-fibre dialyzer can be put to is four.