Decay-accelerating factor (DAF) and complement receptor 1-related gene/protein y (Crry) are two membrane-bound complement regulators on murine erythrocytes that inhibit C3/C5 convertases. Previously, we found that Crry-but not DAF-deficient erythrocytes were susceptible to alternative pathway complement-mediated elimination in vivo. To determine whether it is a unique activity or a higher level expression of Crry makes it indispensable on murine erythrocytes, we over-expressed DAF on Crry-deficient (Crry−/−) erythrocytes by retroviral vector-mediated DAF gene transduction of bone marrow stem cells. DAF retrovirus-transduced erythrocytes expressed 846±127 DAF molecules/cell (DAF high) compared with 249±94 DAF molecules/cell (DAF low) and 774±135 Crry molecules/cell on control mouse erythrocytes. DAF high-Crry−/− erythrocytes were significantly more resistant than either DAF low-Crry−/−, DAF−/−-Crry+/+ or wild-type erythrocytes to classical pathway complement-mediated C3 deposition in vitro. Furthermore, increased DAF expression rescued Crry−/− erythrocytes from acute alternative pathway complement attack in vivo. Notably, long term monitoring revealed that DAF high-Crry−/− erythrocytes were still more susceptible than wild-type erythrocytes to complement-mediated elimination as they had a shorter half-life in complement-sufficient mice but survived equally well in complement-deficient mice. These results suggest that both a high level expression and a more potent anti-alternative pathway complement activity of Crry contributed to its indispensable role on murine erythrocytes. Additionally, they demonstrate the feasibility of using stem cell gene therapy to correct membrane complement regulator deficiency on blood cells in vivo.