Phosphoinositide 3-kinases (PI3Ks) regulate fundamental cellular responses such as proliferation, apoptosis, cell motility, and adhesion. Viable gene-targeted mice lacking the p110 catalytic subunit of PI3Kγ were generated. We show that PI3Kγ controls thymocyte survival and activation of mature T cells but has no role in the development or function of B cells. PI3Kγ-deficient neutrophils exhibited severe defects in migration and respiratory burst in response to heterotrimeric GTP-binding protein (G protein)–coupled receptor (GPCR) agonists and chemotactic agents. PI3Kγ links GPCR stimulation to the formation of phosphatidylinositol 3,4,5-triphosphate and the activation of protein kinase B, ribosomal protein S6 kinase, and extracellular signal-regulated kinases 1 and 2. Thus, PI3Kγ regulates thymocyte development, T cell activation, neutrophil migration, and the oxidative burst.