Preincubation of normal human lymphocytes with human interferon for 18-24 h at 37 degrees C resulted in an increase of the activity of both natural killer (NK) cells and antibody-mediated cytotoxic cells (ADCC). The human myeloid line, K-562, which is highly susceptible to NK cells, was employed. ADCC was assessed with antibody-coated chick erythrocytes as targets. NK cells and ADCC were detected in a 4-hour 51Cr release assay. The magnitude of the enhancement was proportionate to the amount of interferon used in preincubation of the effector cells. Preincubation of tumor-target cells with interferon does not increase their susceptibility or resistance to lysis. The major effect of interferon on the cellular metabolism of the tumor-target cell is inhibition of DNA synthesis, but no direct cytotoxic effect was detected. Our findings may be important in understanding the mode of action of interferon in increasing host resistance to a variety of pathogens and tumors. This may be accomplished by inhibiting the growth of the tumor while simultaneously enhancing the natural killing mechanism for immunosurveillance.