A study was undertaken to assess the ability of the erythrocyte to protect other tissues against oxidative damage. Radiolabelled (51Cr) human umbilical vein endothelial cells (HUVEC) were incubated with erythrocytes and neutrophils activated with phorbol myristate acetate (PMA). Damage to the endothelial cells was indicated by release of radioactivity into the suspending medium. We found that the co-incubation of HUVEC with an increasing range of erythrocyte concentrations resulted in a dose-dependent reduction in the release of radioactivity. When the ability of superoxide to cross the erythrocyte membrane or the glutathione systems was inhibited, the extent of endothelial cell damage increased. Inhibition of the catalase system did not affect results. It was concluded that the erythrocytes afforded some protection against oxidative damage to the endothelial cells by taking up and deactivating the superoxide ions. This protection depends upon intact erythrocyte antioxidant systems. These data support the hypothesis that erythrocytes can provide antioxidant protection to other tissues in vivo.