The combination of 5-fluorouracil (FUra) plus IFN-beta was studied in vitro using a human colon carcinoma cell line, HCT-8. Continuous exposure to high concentrations of IFN-beta is cytotoxic to these cells (ED50, 600 +/- 50 IU/ml). A strong synergism (P < 0.002) was observed when a short-term (1-h), high-concentration exposure to fluoropyrimidine (300 or 1000 microM) was followed by IFN-beta given continuously. In fact, the mean ratio between the expected (product of the survival of each agent alone) and the observed clonogenic cell survival rates of the combination was 3.4 (range, 2.4-4.9). Longer exposures to the fluoropyrimidine (24 h, 7 days) produced less than additive effects with IFN-beta, indicating strong schedule dependency for this synergism. The mechanism of interaction was studied at four levels. First, thymidylate synthase (TS) activity, inhibition, and recovery were measured by an in situ assay in cells treated with FUra, IFN-beta, and their combination. When the prolonged infusion of IFN-beta followed a 1-h exposure to FUra, the observed TS inhibition and recovery, at each time point, were very similar to the expected values, indicating that the interactions between these drugs at the level of TS are not the determinant of the synergism. Second, cell cycle analysis was done. During the continuous exposure to IFN-beta, a significant accumulation of HCT-8 cells in S-phase was observed at 24, 48, and 72 h compared to untreated controls (P = 0.003); however, under the same experimental conditions producing synergy in the clonogenic assay, no significant cell cycle perturbations were produced by the combination of FUra followed by IFN-beta compared to those caused by each agent alone. Third, using the alkaline elution test, we also demonstrated that the synergism is not due to enhanced FUra-induced DNA single-strand break frequency in high molecular weight DNA. Finally, nucleic acid incorporation experiments, using tritiated FUra, showed that the cytokine, given continuously (300 IU/ml), enhanced the amount of FUra incorporated into nucleic acids 24 h after a 1-h exposure to 300 and 1000 microM of FUra. The median percentage of enhancement values were 31.6 +/- 11.5%,m for the lower drug concentration and 18. 4 +/- 8.1% for the higher drug concentration tested. These results suggest that the mechanism of this synergism may be related to the ability of IFN-beta to promote the incorporation of intracellular FUra metabolites into nucleic acids and/or to inhibit the cleavage of FUra nucleotides from RNA/DNA.