When 2-butoxyethanol (2-BE) is administered to rats, hemolysis occurs as the active metabolite butoxyacetic acid (BAA) is formed. Human red blood cells appear to be relatively resistant to the hemolytic effects of BAA in vitro, whereas rat red blood cells undergo changes in deformability, cell swelling, and hemolysis. In this study, exposure of human red blood cells to high concentrations of BAA resulted in loss of deformability and a small increase in mean cellular volume, but no significant hemolysis. These changes resembled the changes that occur in rat erythrocytes exposed to much lower concentrations of BAA. Therefore, a comparison was made between the sub-hemolytic effects of BAA at high concentrations (up to 10 mM) on human red cells with the sub-hemolytic effects of lower concentrations of BAA (up to 0.1 mM) on rat erythrocytes. Under these conditions, human and rat erythrocyte deformability decreased, while mean cellular volume (MCV) and osmotic fragility increased. Although there was a substantial shift in rat erythrocytes to lower densities, human erythrocyte density was only slightly decreased. Human and rat erythrocyte sodium also increased. Rat erythrocytes demonstrated increased spherocytosis. In a survey of blood samples from adults and children, none demonstrated an increase in hemolysis (n = 97) or MCV (n = 65) after exposure to 10 mM BAA for 4 h. In these experiments, in which hemolysis was not evident, human erythrocytes required exposure to a 100-fold greater concentration of BAA to develop changes in red cell deformability, osmotic fragility, and sodium content similar to those observed in rat erythrocytes. These concentrations are not likely to occur under normal human use of 2-BE-containing products.