Mouse milk fat globule epidermal growth factor 8 (MFG-E8), which is secreted by a subset of activated macrophages, binds to apoptotic cells by recognizing phosphatidylserine and promotes their engulfment. Many apoptotic cells are left unengulfed in the germinal centers of the spleen in MFG-E8(-/-) mice, and these mice develop an autoimmune disease resembling human systemic lupus erythematosus (hSLE). Here, we report that hMFG-E8 bound to phosphatidylserine and an integrin alpha(v)beta(3) complex. Increasing concentrations of MFG-E8 generated a bell-shaped response curve for the efficiency of phagocytosis. That is, in NIH3T3 and MFG-E8(-/-) thioglycollate-elicited peritoneal macrophages that do not express MFG-E8, hMFG-E8 enhanced engulfment at low concentrations but inhibited it at high concentrations. On the other hand, hMFG-E8 dose-dependently inhibited the engulfment of apoptotic cells by MFG-E8(+/+) thioglycollate-elicited peritoneal macrophages, indicating that an excess of MFG-E8 has an inverse effect on the engulfment of apoptotic cells. To investigate the role of MFG-E8 in human disease, we generated two mAb against MFG-E8 and screened human blood samples for MFG-E8 using an ELISA. We found that some childhood-onset and adult SLE patients carried a significant level of MFG-E8 in their blood samples. These results suggested that the aberrant expression of MFG-E8 is involved in the pathoetiology of some cases of hSLE.