The potentiation of 5-fluorouracil (5FU) cytotoxicity by folinic acid (FA) is based on strong experimental basis. This experimental concept has been prolonged in the clinical setting where 5FU-FA combination has demonstrated its superiority in comparison to 5FU alone for the chemotherapeutic treatment of advanced colorectal cancer. A justified question concerns the stability for 5FU (new French preparation, with sodium hydroxyde, by Roche)-FA admixtures studied in clinically compatible conditions. We have presently undertaken this study. Based on drug concentrations corresponding to current 5FU-FA chemotherapeutic protocols, we have tested the influence of various factors like the vials, diluents and the type of FA preparation (Lederfoline versus Elvorine). The mixture between 5FU and FA leads to the formation of a precipitate which is influenced by the contact time between 5FU and FA (threshold at 24 hr), by the relative concentration of each drug in mixture and its seems necessary to avoid the mixture between undiluted 5FU and FA (d,1 preparation, Lederfoline). Cassettes or plastic bags are preferable to glass recipients. Finally and above all, the use of pure lFA form (Elvorine) does not lead to the formation of a precipitate seen with the d'1 FA form (Lederfoline) at equal concentration of active folate (1 form). These data carry practical importance for the security and the quality of 5FU-FA protocols and particularly when using portable pumps during prolonged periods necessitating the use of small volume of injectable drug solutions.