Host defense mechanisms against cancer depend on an intact cellular immunity system. Immunosurveillance depends on thymus lymphocytes which, when sensitized, form lymphokines. One of the important lymphokines produced by T-lymphocytes is called interferon, well known for its antiviral effects. Recent experimental evidence points also to the potential effectiveness of interferon against malignancies. Interferon and interferon-inducers have been found to alter the course of solid tumors, leukemia, sarcomas and lymphomas in experimental animals, possibly by stimulating the reticuloendothelial system to produce tumor rejection or by altering the surface of cells to change tumor and host reactions.