The appearance of phosphatidylserine (PS) on the cell surface during apoptosis in thymocytes and cytotoxic T lymphocyte cell lines provokes PS-dependent recognition by activated macrophages. Flow cytometric analysis of transbilayer lipid movements in T lymphocytes undergoing apoptosis reveals that downregulation of the adenosine triphosphate-dependent amino-phospholipid translocase and activation of a nonspecific lipid scramblase are responsible for PS reaching the surface from its intracellular location. Both mechanisms are expressed at the same time, and precede DNA degradation, zeiosis, and cell lysis in the apoptotic pathway.