Phospholipid transfer protein (PLTP) exists in a high-activity (HA-PLTP) and a low-activity form (LA-PLTP) in the circulation. LA-PLTP is associated with apoA-I while the HA-PLTP complex is enriched with apoE. To study the interaction of PLTP with apolipoproteins, we carried out surface plasmon resonance analyses. These demonstrated a concentration-dependent binding of recombinant human PLTP, which represents an active PLTP form, and LA-PLTP to apoE, apoA-I, and apoA-IV within a nanomolar K(D) range. To study whether LA-PLTP can be transformed into an active form, we incubated it in the presence of proteoliposomes containing apoE, apoA-I or apoA-IV. The apoE proteoliposomes induced a concentration-dependent activation of LA-PLTP. ApoA-IV proteoliposomes also activated LA-PLTP in a concentration-dependent manner, whereas apoA-I proteoliposomes had no such effect. These observations suggest that PLTP is capable of interacting with apoE, apoA-I, and apoA-IV, and that these interactions regulate PLTP-activity levels in plasma.