Profilin is a conserved, widely distributed actin monomer binding protein found in eukaryotic cells. Mammalian profilin reversibly sequesters actin monomers in a high affinity profilactin complex. In vitro, the complex is dissociated in response to treatment with the polyphosphoinositides, phosphatidylinositol monophosphate, and phosphatidylinositol 4,5-bisphosphate. Here, we demonstrate the ultrastructural immunolocalization of profilin in human leukocytes and platelets. In both cell types, a significant fraction of profilin is found associated with regions of cell membrane devoid of actin filaments and other discernible structures. After platelet activation, the membrane association of profilin reversibly increases. This study represents the first direct evidence for an interaction between profilin and phospholipids in vivo.