In order to investigate the combined antitumor activity of 5-fluorouracil (5-FU), and recombinant human interferon alpha 2a (IFN alpha) or human fibroblastoid interferon beta (IFN beta), the 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2H tetrazolium bromide (MTT) assay was carried out using a cultured human colon cancer cell line (C-1) and fresh surgical specimens of gastric and colon carcinomas. IFNs did not show positive antitumor activity against C-1 cells, whereas 5-FU showed time- and concentration-dependent antitumor activity against C-1 cells. Furthermore, the antitumor activity of 5-FU on C-1 cells was augmented by IFN alpha or beta. When 5-FU (50 micrograms/ml) with IFN alpha (50 IU/ml). or IFN beta (50 IU/ml) was applied for the MTT assay with 48 hours incubation of fresh surgical specimens of gastric and colon carcinomas, the inhibition rates increased by 10% in 9 of 21 gastric specimens and in 18 of 36 colon carcinomas for IFN alpha (47.4% or 27/57), and in 8 of 15 gastric specimens and in 15 of 28 colon carcinomas for IFN beta (53.5% or 23/43). These results suggest that the chemosensitivity to 5-FU of human gastric and colon carcinomas is increased in the presence of IFNs, without involvement of the host-mediated immune system, and that this combined effect can be predicted by the MTT assay in vitro.