Air pressure decay (APD) rate and ultrafiltration rate(UFR) tests were performed on new and saline rinsed dialyzers as well as those reused in patients several times. C-DAK 4000 (Cordis Dow) and CF 15-11 (Baxter Travenol) reused dialyzers obtained (rom the dialysis clinic were used in the present study. The new dialyzers exhibited a relatively flat APD, whereas saline rinsed and reused dialyzers showed considerable amount of decay. C-DAK dialyzers had a larger APD($11.70{pm}1.32$mmHg/min) compared to CF dialyzers($4.32{pm}0.55$mmHg/min) (p<0.05). However, there was no observable difference in the UFR between the two dialyzers. Neither APD nor UFR showed any significant increase with an increasing number of reuses for up to more than 20reuses. A substantial number of failures observed in APD(larger than 20mmHg/min)on the reused dialyzers(2 out of 40 CF and 5 out 26 C DAK) were attributed to the possible damage on the fibers. The CF 15-11 HFDs which failed APD test did not show changes in the UFR compared to normal dialyzers indicating that APD Is a more son sitive test than UFR test to evaluate the integrity of the fibers.