The authors clarified the characteristics of effector cells of which inhibitory activity against the tumor cell growth was enhanced by interferon (IF). These cells adhered to nylon wool, but they did not adhere to a glass surface. They showed the EA-rosettees formation, but they did not show the E-rosett formation. This activity was diminished by trypsinization, X-ray irradiation and treatment with hydrocortisone and progesterone. It was not changed by the addition of aggregated Ig. In summary, the effector cells in the present study could not be identical to NK cells.