Abstract The interaction between macrophage signal-regulatory protein alpha (SIRPα) and its ligand CD47 serves as a critical self-recognition mechanism, in which their interaction triggers inhibitory signaling via SIRPα cytoplasmic ITIMs and prevent macrophage phagocytosis toward self-cells. Despite this mechanism being suggestively imperative, CD47−/− mice demonstrate no, or only mild, macrophage phagocytosis toward self-cells, suggesting additional activation and inhibitory mechanisms controlling macrophage behavior. Studying our recently established SIRPα −/− mice, as well as CD47−/− mice, we identify that macrophage phagocytosis toward self cells requires activation by inflammatory cytokines/factors. Ex vivo phagocytosis assays demonstrate that macrophages from SIRPα −/− or CD47−/−, and even WT mice, are generally incompetent to attack self-cells, which bear no ‘eat-me’ signal and thus do not activate macrophage phagocytosis. However, treating these macrophages with IL-17, IL-6, IL-1β, TNFα and TLR agonists, but not IFNγ, dramatically initiates potent phagocytosis toward self-cells, for which only the CD47-SIRPα interaction restrains. Mechanistic studies suggest that a PKC-Syk-mediated signaling pathway, to which IL-10 conversely inhibits, is required for activating macrophages toward self cells. Application of activated SIRPα −/− macrophages in targeting CD47-expressing cancer cells also display potent eradication of B16, LLC, MC38, and EL4 cancer cells. These findings significantly expand our knowledge of macrophage phagocytic plasticity under different conditions and also provide insights into strategies for enhancing transplantation tolerance and macrophage-based cancer eradication.