Previous studies have shown that 5-hexyl-2'-deoxyuridine (HdUrd) blocked the cytotoxic effects of 5-fluorodeoxyuridine and deoxyadenosine in L1210 cells. HdUrd had no effect in preventing the inhibitory effects of 5-fluorouracil. These data suggested that HdUrd was an inhibitor of nucleoside transport in L1210 cells (Cory, J. G.; Halley, M. C.; Janey, A.; Lapis, K. Cancer Res. 50:4552-4556; 1990). Studies have now been carried out which show that HdUrd inhibits nucleoside transport as measured by [3H]uridine or [3H]formycin B transport into L1210 cells in culture. The IC50 for HdUrd inhibition of total [3H]uridine uptake was approximately 20 microM in wild-type L1210 cells. Since wild-type L1210 cells have three distinct nucleoside transporters, the effect of HdUrd on each transporter was examined using the non-metabolized nucleoside analog, formycin B. The nitrobenzylmercaptopurine riboside (NBMPR)-sensitive transporter, es, was most sensitive to HdUrd with an IC50 of 1.0 +/- 0.1 microM; the NBMPR-insensitive transporter, ei, was much less sensitive to HdUrd with an IC50 of 32 +/- 2 microM; the sodium ion-dependent transporter, cif, was the least sensitive transporter to HdUrd with an IC50 of 130 +/- 5 microM. These data support the concept that HdUrd, a relatively non-cytotoxic agent, could be useful in increasing the potency of antitumor inhibitors directed at the de novo pathways for nucleotide synthesis through the blockage of the salvage pathways for nucleosides.