Many works have reported the interaction between selenium and mercury in the mammalian body and that chalcogen seems to have a protective effect against mercury toxicity. The aim of this study was to investigate the hemolytic effects of sodium selenite and/or mercuric chloride in human blood under in vitro conditions. For this, total venous blood from healthy subjects (males and females) was heparinized and incubated at 37 degrees C for 90 min with different concentrations of sodium selenite and/or mercuric chloride. The hemolytic effects of compounds were evaluated by measuring plasma hemoglobin concentration after centrifugation. In addition, 2-thiobarbituric acid reactive substances (TBARS) from plasma and erythrocytes, as well as erythrocyte nonprotein thiols (NPSH), were also evaluated in order to investigate molecular mechanisms related to selenite- or mercury-induced hemolysis. Mercuric chloride and sodium selenite, alone (400 microM), promoted a small in vitro hemolytic effect in human erythrocytes. However, when blood was exposed to both compounds (200 microM of each), there was an extremely high synergistic hemolytic effect. The exposure of blood to sodium selenite (400 microM), mercuric chloride (400 microM), and both compounds (200 microM each) did not alter erythrocyte TBARS levels. Sodium selenite presented a high oxidant effect toward erythrocyte NPSH; however, this effect was inhibited by mercuric chloride. The current results point to a synergistic hemolytic effect of sodium selenite and mercuric chloride in human blood, suggesting new understanding on the selenium-mercury antagonism. Moreover, this observed hemolysis seems to be not related to lipoperoxidation or thiol depletion.