In vitro results have clearly demonstrated that leucovorin (LV) can enhance the growth inhibitory effects of 5-fluorouracil (5FU) but in vivo potentiation of the antitumor effect of 5FU by LV has not yet been defined in animal models. The antitumor effect and the toxicity of the LV-5FU combination was studied in mice bearing the colon carcinomas, Colon 26 and Colon 38. Mice were treated weekly with 5FU at the maximum tolerated dose (100 mg/kg) alone or with LV at different doses and schedules. Pretreatment with LV followed 1 hr later by a second LV injection together with 5FU clearly potentiated the antitumor effect of 5FU in both murine tumor lines. Comparable results were obtained with total LV doses of 100 and 200 mg/kg. The effect of 5FU pretreatment was studied by randomization of 5FU pretreated Colon 38-bearing mice in 2 groups, one treated with 5FU and the other with LV-LV + 5FU. Again, LV potentiated the effect of 5FU. Also in a Colon 38 tumor which had developed resistance against 5FU and which was reimplanted, LV potentiated the antitumor activity of 5FU. Weight loss of the combination was slightly higher than for 5FU alone. A moderate leukopenia (nadir 40%) and mild anemia were observed, which were less than for 5FU alone. The combination did not cause thrombocytopenia. In conclusion, LV can potentiate the therapeutic efficacy of 5FU in murine colon carcinoma.