The combination of 5-fluorouracil-folinic acid and oxaliplatin has led to a significant improvement of chemotherapy efficacy in advanced pretreated colorectal cancer. The objective of the present study was, considering the oxaplatin-5-fluorouracil-folinic acid combination, to examine the impact of one given drug on the cellular determinants of cytotoxic activity of the other drug. These cellular factors were analysed on the human colon cancer cell line WiDr in clinically relevant conditions of drug exposure ('De Gramont' schedule) with oxaliplatin-folinic acid during 2 h followed by 5-fluorouracil 48 h. The DNA binding of oxaliplatin was significantly reduced by the presence of 5-fluorouracil but this effect was time-dependent and after 50 h the platinum incorporated into DNA was identical in controls and in the drug combination. In the presence of oxaliplatin, there was less formation of FUH(2) which is the first catabolite produced in the cascade of 5-fluorouracil metabolic degradation. The effects of drugs on cell cycle were quite different from one drug to the other with oxaliplatin inducing a shift towards G(2) accumulation and 5-fluorouracil-folinic acid to a greater proportion of cells in G(1)-S. When oxaliplatin and 5-fluorouracil-folinic acid were combined the cell cycle effects were very similar to that of the 5-fluorouracil-folinic acid sequence alone. Oxaliplatin was able to reduce thymidylate synthase activity with a marked impact 28 h after the beginning of cell exposure to the drug. The 5-fluorouracil-folinic acid drug sequence led to a profound reduction in thymidylate synthase activity and this decrease was not markedly enhanced by the presence of oxaliplatin. Regarding apoptosis, changes in mitochondrial membrane permeability were observed in the presence of the tested drugs and the impact of 5-fluorouracil-folinic acid was greater than that of oxaliplatin. The addition of oxaliplatin did not amplify the action of 5-fluorouracil-folinic acid upon mitochondrial membrane permeability change. The presence of oxaliplatin itself did not modify the intracellular concentration of total reduced folates. The fact that oxaliplatin may reduce 5-fluorouracil catabolism could be central in explaining the supra-additive interaction between these drugs.