Tumor progression is characterized by several hallmarks, including growth signal independence, inhibition of apoptosis, and evasion of the immune system, among others. There are many well-documented examples of changes that occur in tumor cells to allow escape from the immune system. In addition, a large body of evidence indicates that mice that lack functional B, T, or natural killer cells, or that have these cells depleted experimentally, have higher rates of tumor incidence than normal mice. It has also been observed that macrophages frequently infiltrate tumors and can modulate T-cell and stromal activity to either favor or inhibit tumor growth. However, there is a relative dearth of evidence supporting the tumoricidal activity of macrophages independently of their effects on other cells. Over the last decade, a marker of self that protects an organism's own cells from macrophage clearance has been discovered and studied; the protein, CD47, interacts with a receptor on macrophages to prevent phagocytosis. We have found that CD47 is up-regulated in myeloid leukemia in mouse and man, even at the level of primitive stem and progenitor tumor cells, indicating that evasion of macrophage phagocytosis might be important in the evolution of these cancers. In support of this hypothesis, we have found that ectopic expression of CD47 on a leukemia cell line allows the cells to grow in mice that are T, B, and NK-cell deficient, whereas they are otherwise cleared rapidly. We also provide evidence that CD47 functions to inhibit phagocytosis of these tumor cells by macrophages in dose-dependent manner. CD47 may play a role in non-neoplastic hematopoiesis as well. Following a large stress on the immune system, hematopoietic stem and progenitor cells expand their numbers and move to the periphery to initiate hematopoiesis in distant organs such as the spleen and liver. Macrophages seem to be particularly primed for phagocytosis during these states as well, putting the organism's own cells at risk during this mobilization. We have found that CD47 is dramatically up-regulated in stem and progenitor cells during such a stress response, and that the levels drop after the inflammatory state abates. The level of CD47 expressed by the stem and progenitor cells during such a state determines the probability of clearance by macrophages. Thus, we conclude that CD47 up-regulation is a physiologically relevant phenomenon in states where hematopoietic cells are at risk for being phagocytosed. Furthermore, leukemic cells are able to upregulate CD47 as well, ensuring that they are not recognized as foreign targets that need to be cleared by activated macrophages. In total, these studies illustrate the importance of phagocytosis in hematopoeisis, in both normal hematopoietic cells that are at risk in macrophage rich sites and in tumor cells that are able evade this novel type of tumor immunosurveillance.