Macrophages play an essential role in defending against invading pathogens by migrating to the sites of infection, removing apoptotic cells, and secreting inflammatory cytokines. The molecular mechanisms whereby macrophages regulate these processes are poorly understood. Using bone marrow–derived macrophages (BMMs) deficient in the expression of p85α-subunit of class IA phosphatidylinositol 3 (PI-3) kinase, we demonstrate 50% reduction in proliferation in response to macrophage–colony-stimulating factor (M-CSF) as well as granulocyte macrophage–colony-stimulating factor (GM-CSF) compared with wild-type controls. Furthermore, p85α^–/– BMMs demonstrate a significant reduction in migration in a wound-healing assay compared with wild-type controls. The reduction in migration due to p85α deficiency in BMMs is associated with reduced adhesion and directed migration on fibronectin and vascular cell adhesion molecule-1. In addition, deficiency of p85α in BMMs also results in defective phagocytosis of sheep red blood cells. Biochemically, loss of p85α in BMMs results in reduced activation of Akt and Rac, but not Erk (extracellular signal-related kinase) mitogen-activated protein (MAP) kinase. Taken together, our results provide genetic evidence for the importance of p85α in regulating both actin- and growth-based functions in macrophages, and provide a potential therapeutic target for the treatment of diseases involving macrophages, including inflammation.