Exponentially growing HeLa cells were exposed to thymidine (dThd), 5-fluorouracil (FUra), and deoxycytidine (dCyd) singly and in combination for up to 3 days. Survivals were measured using a colony forming assay, and cytokinetic effects of the drugs were evaluated by flow cytometry and by determination of population growth rates. Increasing concentrations of dThd alone (up to 4 mM) progressively slowed the movement of cells through the cycle; dThd was toxic above 1 mM. Nontoxic levels of FUra (10(-5) M) inhibited cell progression, but this was abolished by very low concentrations (3 microM) of dThd. The cytotoxic effects of FUra were not reversed by dThd; in fact, the lethality of FUra was enhanced in some cases. dCyd alone had neither cytotoxic nor cytokinetic effects on HeLa cells up to 4 mM, but low concentrations reversed the cytotoxic effects of 4 mM dThd and high concentrations reversed its cytokinetic effects. When dCyd was added to the combination of dThd and FUra, synergistic lethality resulted even at 10(-5) M FUra. The mechanism by which this synergism was achieved was not simply increased incorporation of FUra into RNA since, while dThd did enhance this incorporation, dCyd did not further increase it. Our results suggest instead that dCyd either directly or indirectly inhibited the repair of FUra-associated damage or else enabled the cells to progress into a more sensitive phase of the cell cycle during exposure.