Iododeoxyuridine (IdUrd) potentiated the lethal but not the growth inhibitory properties of fluorouracil (FUra) and fluorodeoxyuridine (FdUrd) in human bladder cancer cells (T24). The rate of incorporation of IdUrd into DNA was enhanced by both fluoropyrimidines, but to a significantly greater extent by FdUrd. Both inhibition of iododeoxyridylate dehalogenation and the depletion of thymidine triphosphate pools contributed to the increased incorporation rate. Inhibition of dehalogenation accounted for 67% of the observed stimulation in the case of FUra, but only 37% of the increase produced by FdUrd. The depletion of dTTP pools, both in the presence and absence of IdUrd, was greater after FdUrd than FUra exposure. The observed increase in the rate of incorporation of IdUrd appears to account for the enhanced toxicity seen with FdUrd, but other factors may be involved in the case of FUra. Since FUra and IdUrd appear to be mutually potentiating and do not share a dependence on thymidine kinase activity, this drug combination warrants further investigation.