There is evidence that the polymerization of actin takes place at the plasma membrane, and that profilactin (profilin/actin complex), the unpolymerized form of actin found in extracts of many non-muscle cells, serves as the immediate precursor. Both isolated profilin and profilactin interact with detergent when analysed by charge shift electrophoresis, indicating that they have amphipathic properties and may be able to interact directly with the plasma membrane. We demonstrate here that isolated profilin, as well as the profilactin complex, interacts with anionic phospholipids. Phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) was found to be the most active phospholipid, causing a rapid and efficient dissociation of profilactin with a concomitant polymerization of the actin in appropriate conditions. These and other observations suggest the possibility of a relationship between the induction of actin filament formation and the increased activity in the phosphatidylinositol cycle seen as a result of ligand-receptor interactions in various systems.