2-Euryfuryl-1,4-naphthoquinone C1 and its 5- and 5,8-hydroxy derivatives C2 and C3, were tested for their cytotoxicity towards transplantable liver tumor (TLT) cells (a murine hepatoma cell line) in the absence and in the presence of vitamin C. Cell death, caspase-3 activity and two metabolic end-points, namely the intracellular content of ATP and glutathione (GSH), were employed to evaluate their cytotoxicity. In a range of concentration from 0 to 10 μg/ml C1 and C3 were non toxic against TLT cells, while compound C2 killed about 50% of cells by necrosis. Interestingly, the presence of vitamin C did not enhance the cytolysis of C2, but its addition exacerbated the effects of the three compounds on both ATP and GSH contents, the two metabolic end points selected in our study. Our assumption is that the electron donor effect of the peri-hydroxyl substituents on euryfurylnaphthoquinones and the hydrogen bond between the peri-hydroxy and quinone carbonyl groups influence the electronacceptor capability of the quinone nucleus and thus modifies the electron transfer from ascorbate to the electroactive quinone nucleus. The combination of euryfurylnaphthoquinones with vitamin C may be of potential clinical interest, because cancer cells accumulate vitamin C, they are sensitive to an oxidant insult and they depend on glycolysis (ATP formation) for their survival. © 2009 Pharmaceutical Society of Japan.