Menadione (vitamin K3), a redox cycling quinone, is an effective cytotoxic drug used for the treatment of leukaemia. Quercetin is a natural flavonoid with both antioxidant and pro-oxidant properties. We investigated the effects of menadione and quercetin on apoptosis induction and delayed luminescence (DL) of human leukaemia Jurkat T cells following treatment with various doses of the oxidant agent menadione, in the presence or absence of the bioflavonoid at different concentrations. A consistent decrease in DL intensity is observed in both menadione-and/or quercetin-treated cells, suggesting that superoxide anions and inhibition of the mitochondrial electron transport chain alter significantly the DL emission in this cell type. Quantification of these effects can help answer the question on the origin of DL, which at the moment is still a matter of debate.