Red cells stored either for 3 days or 2 weeks were equally susceptible to lysis during in vitro dialysis. Furthermore, the small increases in plasma hemoglobin during hemodialysis of 10 patients were independent of transfusion. We examined the patients during three separate hemodialyses: one without transfusion, one with transfusion of three units of red cells stored for less than 5 days, and one with transfusion of red cells stored for 2 weeks. The mean concentration of hemoglobin in plasma increased from 0.7 to 3.6 mumol per l (p less than 0.01), and it was normal again after 2 days. No significant changes occurred in the mean concentration of haptoglobin in plasma. The results indicate that red cells stored for 2 weeks are not more prone to lysis than those stored for less than 5 days. Consequently, we recommend a storage interval of less than 2 weeks, instead of 5 days, for blood transfused to chronic hemodialysis patients.