Using malondialdehyde (MDH) as an indicator of lipid peroxidation it was examined, whether oxygen radicals could be an origin of freeze-induced weakness of HES-cryopreserved erythrocytes. Each of 11 erythrocyte suspensions (Hct = 40; HES 200,000/0.62/12.5%; 60 mmol NaCl) was separated into 40 ml samples, cooled down to -196 degrees C und stored at -80 degrees C, finally. Samples were thawed after 1, 2, 3 und 6 months storage and besides that, one sample having remained at -196 degrees C (LN2). The MDH content (1.5 mumol/l Ery unwashed; 0.4 mumol/l Ery washed) amounted to 3.4 mumol/l Ery after LN2 storage, to 4 mumol/l Ery after 1 und to 8 mumol/l Ery after 6 months at -80 degrees C. Similarly, the MDH generation rate at -80 degrees C increased with storage time. The membrane fragility (1 in freshly drawn erythrocytes; 1.3 in erythrocytes out of LN2) rose from 1.6 after 1 month to 2.4 after 6 months. MDH content and membrane fragility were correlated linearly (r = 0.98). It is concluded that increased superoxide formation is mediated by freezed-induced oxidation of Hb-bound Fe. This allows peroxidation of membrane lipids which in consequence causes hemolysis.