We describe a novel cell type, the Pinocchio cell, that appears in the peripheral blood of all patients receiving treatment with interleukin 2, up to 20,000 cells/microliter. This cell is characterized by a prominent and granular proboscis with which it attaches to tumor cells and mediates tumor cell lysis. Pinocchio cells are immunologically heterogeneous and express antigens of both T and NK cells; Pinocchio cells are adherent in culture and are more cytolytic than non-adherent cells against NK-sensitive and resistant tumor cells. Incubation of normal whole human blood for 1 h induces Pinocchio morphology of mononuclear white blood cells.