We determined the long-term survival of red blood cells collected postoperatively from the surgical drains, filtered and autotransfused with the Constavac Blood Conservation System. 10 patients with knee arthrosis were treated with cementless total knee arthroplasty and postoperatively connected to the autotransfusion system. Shed blood was collected for 6 hours postoperatively and then reinfused. Before reinfusion, a fraction of the blood shed was radiolabeled with chromium-51 (51Cr). For a postoperative minimum period of 40 days the activity of 51Cr was measured in frequent venous blood samples. The time from 100% to 50% activity of the isotope (T50Cr) was 21 days, equal to that reported for banked autologous blood.