CD93 is emerging as a novel regulator of inflammation. CD93 was identified and cloned based on its ability to modulate C1q-triggered enhanced phagocytosis. However, both CD93 and another putative C1q receptor, LRP, were not required for the C1q-triggered enhancement of phagocytosis; therefore, the C1q receptor and the molecular function of CD93 remained elusive. CD93 belongs to the group XIV family of transmembrane glycoproteins, implicated in regulating inflammation and innate immunity. The molecular structure and expression profile of CD93 suggest a role in adhesion and inflammation. CD93 exists as a membrane bound and truncated soluble form (sCD93), however the bioactive form of CD93 is unknown. Interestingly, CD93-/- mice have a defect in clearance of apoptotic cells, a phenotype associated with dysregulated inflammation; therefore, an investigation was undertaken that tested the hypothesis that CD93 regulated inflammation. CD93 was shed from murine inflammatory macrophages, and sCD93 was elevated with inflammation in vivo suggesting that CD93 may be a biomarker for inflammation. In addition, inflammatory peritoneal lavage fluid from CD93-/- mice failed to enhance the engulfment of apoptotic cells as efficiently as wildtype fluid. These data suggested that the inflammatory microenvironment in the peritoneum of CD93-deficient mice was altered compared to wildtype controls. In support of these observations, CD93-/- mice had a proinflammatory phenotype; CD93-/- mice had 1.6 to 1.8 times more leukocytes in the peritoneum between 3 and 24 hours post injection of thioglycollate compared to wildtype controls. Increased leukocyte recruitment was accompanied by changes in vascular integrity and dysregulated C1q hemolytic activity. sCD93 was elevated with inflammation when CD93 was expressed on non-hematopoietic cells, however low/undetectable levels of sCD93 were found when CD93 was expressed exclusively on hematopoietic cells in bone marrow chimeric mice. Expression of CD93 on either hematopoietic or non-hematopoietic cells restored leukocyte recruitment and C1q hemolytic activity to wildtype levels, suggesting the cell-associated form was required to regulate inflammation. In conclusion, these data demonstrate that cell-associated CD93 is required for normal leukocyte recruitment and C1q hemolytic activity during peritonitis, and support the hypothesis that CD93 is required for the regulation of acute inflammation and innate immunity.