A thiazine dye reductase has been described in endothelial cells that reduces methylene blue (MB), allowing its uptake into cells. Because a different mechanism of MB uptake in human erythrocytes has been proposed, we measured MB uptake and reduction in this cell type. Oxidized MB (MB(+)) stimulated reduction of extracellular ferricyanide in a time- and concentration-dependent manner, reflecting extracellular reduction of the dye. Reduced MB was then taken up by the cells and partially oxidized to MB(+). Both forms were retained against a concentration gradient, and their redox cycling induced an oxidant stress in the cells. Whereas concentrations of MB(+) <5 microM selectively oxidized NAD(P)H, higher concentrations also oxidized both glutathione (GSH) and ascorbate, especially in the absence of d-glucose. MB(+)-stimulated ferricyanide reduction was inhibited by thiol reagents with different mechanisms of action. Phenylarsine oxide, which is selective for vicinal dithiols in proteins, inhibited MB(+)-dependent ferricyanide reduction more strongly than it decreased cell GSH and pentose phosphate cycle activity, and it did not affect cellular NADPH. Open erythrocyte ghost membranes facilitated saturable NAD(P)H oxidation by MB(+), which was abolished by pretreating ghosts with low concentrations of trypsin and phenylarsine oxide. These results show that erythrocytes sequentially reduce and take up MB(+), that both reduced and oxidized forms of the dye are concentrated in cells, and that the thiazine dye reductase activity initially responsible for MB(+) reduction may correspond to MB(+)-dependent NAD(P)H reductase activity in erythrocyte ghosts.