Referring to synergistic antitumor effects of natural human tumor necrosis factor (n-TNF) derived from human acute lymphoblastic leukemia BALL-I cell as well as mouse interferon beta (mIFNbeta) and mouse interferon gamma (mIFNgamma), a series of the study was made using Lewis lung carcinoma grafted on BDF1 mice. With a combination dose of n-TNF (1 x 10^2 U/kg/day) and mIFNbeta (1 x 10^2 IU/kg/day) as well as that of n-TNF (1 x 10^2 U/kg/day) and mIFNgamma (1 x 10^2 IU/kg/day), a significant enhancement of antitumor effect was observed. Furthermore, with a triple combination dose of n-TNF (1 x 10^2 U/kg/day), mIFNbeta (I x 10^2 IU/kg/day) and mIFNgamma (I x 10^2 IU/kg/day), too, a strong synergistic effect was noted. The concentration of n-TNF required for concomitant use with mIFNbeta and mIFNgamma was 1 over 5 x 10^3 of that required for single dose of n-TNF, to obtain the same level of effect.