From 1986 to 1996 80 children (46 males and 34 females) 1-15 years old with aplastic anemia (AA) and myelodysplastic syndrome (MDS) had been observed. All children lived in the regions with chronic action of low doses radiation. 41.6% of children have increased lead (Pb) plasma level, 29.1% - mercury (Hg) and 18.7% - both of them. Enzyme and non-enzyme antioxidant protective systems(AOPS) (superoxide dismutase (SOD), erythrocyte catalase (EC), glutatione peroxidase (GP), reduced glutatione) and lipid superoxidation (LSO) in erythrocytes and serum (products of thiamine barbital acid) were studied. Enzyme AOPS disturbances in all patients with AA and MDS were revealed. Intentification of free radical processes had place both due to the decries of protective qualities SOD and higher level serum ferritin. Changes of EC activity were different. In comparison with control group and patients with congenital AA the increase of erythrocyte GP activity in patients with acquired AA and MDS had been revealed. Marked SOD changes, increase of EC activity and GP we established in patients with higher serum Pb level and Pb+Hg levels. As compared to the control group processes of LSO were significantly intensified. It testifies about activation of free radical products. These results show the reduction of erythron AOPS. Thus, combined action Pb and Hg, free-radical products in children with AA and MDS from regions with low-doses radiation decreases erythron antioxidant protest system and results in erythron structure and functional disturbances. It is one of pathogenetic mechanisms AA and MDS on molecular cell level.