Augmentation of anti-tumor effect of interleukin 2 (IL-2) with sizofiran (SPG) was demonstrated with in vivo and in vitro experiments. C3H/He mice with subcutaneously inoculated X5563 tumor were used as experimental models. IL-2 at a dose of 2 X 10(4) units per mouse by subcutaneous injection, and/or SPG at a dose of 0.1 g per mouse by intramuscular injection were given every other day for a total of ten times. Tumor infiltrating lymphocytes (TIL) were investigated with the avidin-biotin peroxidase complex method. Cytotoxic activity of mice spleen cells after the therapies against YAC-1, X5563 and MH134 were tested with 51Cr release assay. IL-2 or SPG alone suppressed tumor growth in vivo, although not significantly. Combination therapy with IL-2 and SPG suppressed tumor growth in vivo significantly (p = 0.04). Mice treated with the combination survived longer than the mice treated with the single drug (p less than 0.05) and the controls (p less than 0.001). Immunohistologically, more TILs were seen in the combination group than the other groups. In the cytotoxicity study, 3 days after initiation of the therapies, augmentation of natural killer (NK) activity was greater by the combination than by IL-2 or SPG alone. After incubation with IL-2, the spleen cells from mice treated with the combination showed higher cytotoxicity against X5563 or MH134 tumors than those treated with the single drug or controls. Obtained results suggested that combination therapy with IL-2 and SPG effectively induced NK cells and lymphokine-activated killer cells in vivo and may lead to greater clinical benefit in the treatment of malignancies.