We investigated the in vivo antitumor effect of recombinant human interferon-α,γ (IFN-α,γ) and/or recombinant human tumor necrosis factor-α (TNF-α) against human colon cancer cells (LoVo) transplanted into nude mice. LoVo cells were very sensitive to IFN-γ and slightly sensitive to IFN-α and resistant to TNF-α in vitro by dye uptake method. Intravenous administration of IFN-γ significantly inhibited tumor growth transplanted subcutaneously into nude mice, but IFN-α and/or TNF-α did not show any antitumor effect. Labeling index on stanining with bromodeoxyuridune and mitotic index of LoVo cells treated with IFN-α,γ or TNF-α differed slightly from those of the control group. We previously reported that the synergistic antitumor effects on the three cell lines could be examined by the combined use of IFN-α and TNF-α and the mechanism of the synergism was arrested in the S phase of cell cycle of target cells. However in the cese of LoVo cells, the combined use of IFN-α and TNF-α was not effective and did not arrest the cells in tha S phase of cell cycle. Therefore, arrest in the S phase was suspected to be responsible from the synergistic antitumor activity of IFN-α and TNF-α on the sensitive targets.