Potentiation of the anti-tumor activity of 5-fluorouracil (5FU) by methotrexate (MTX) and leucovorin (LV) requires careful scheduling to achieve the most favorable interactions between these 3 drugs. In vitro the triple combination of MTX/5FU/LV was tested at 4 time intervals and with 3 MTX concentrations in 3 cell lines. MTX/5FU/LV was never superior to both 5FU/LV and MTX/5FU, moreover LV reduced growth inhibition by MTX/5FU in 2 cell lines. MTX/5FU was associated with only moderately increased levels of the 5FU metabolite FdUMP, while LV/5FU reduced the free FdUMP levels. MTX/LV/5FU had a dual effect and FdUMP levels gave no clear indication regarding the anti-proliferative effect that could be expected. In vivo the time-dependent potentiation of MTX on the 5FU anti-tumor activity was confirmed for the treatment of HNX-14C-bearing mice. Addition of LV to the MTX/5FU combination failed to cause a gain in anti-tumor efficacy. For the treatment of Colon-26-bearing mice, MTX/5FU was comparable to LV/5FU, but the triple combination of MTX/LV/5FU seemed to have a better anti-tumor effect. However, the toxicity of the latter treatment was greater than that of LV/5FU. The data from both in vitro and in vivo experiments failed to support the addition of LV to MTX/5FU therapy, possibly due to a reversing effect of LV.