Although the thymidine analogues 5-bromo-2'-deoxyuridine (BrdUrd) and 5-iodo-2'-deoxyuridine (IdUrd) have been used successfully as radiation sensitizers in clinical trials, it is not clear which of these agents is the more promising to pursue. To begin to assess this question with regard to colorectal cancer metastatic to the liver, a study was carried out using HT29 human colon cancer cells in culture and implanted in nude mice as xenografts. Cells and animals were treated with BrdUrd +/- the thymidylate synthase inhibitor 5-fluoro-2'-deoxyuridine (FdUrd), and the results compared to our previous studies with IdUrd +/- FdUrd (T. S. Lawrence, M. A. Davis, P. E. McKeever, J. Maybaum, P. L. Stetson, D. P. Normolle, and W. D. Ensminger. Cancer Res., 51: 3900-3905, 1991). Using cultured cells, it was found that FdUrd (at concentrations of greater than 10 nM) increased: (a) the incorporation of BrdUrd into the DNA of cultured tumor cells; (b) BrdUrd-mediated radiosensitization; (c) BrdUrd-mediated increase in radiation-induced DNA damage; and (d) BrdUrd-mediated decrease in the repair of radiation-induced damage. The incorporation of BrdUrd was greater than or equal to the incorporation of IdUrd previously determined under the same exposure conditions. Studies using nude mice bearing HT29 xenografts showed that FdUrd increased BrdUrd incorporation more into tumors than into the normal liver. Most tumor cells incorporated BrdUrd (labeling index after a 4-day infusion = 87 +/- 2%; SE); in the liver, labeling was confined chiefly to nonparenchymal cells. In both the presence and absence of FdUrd, the incorporation of BrdUrd into tumors was significantly and consistently greater than the incorporation of IdUrd measured under the same conditions of drug administration (by a factor of 1.2-3.6). Furthermore, the administration of BrdUrd +/- FdUrd tended to produce less weight loss and hematological toxicity than IdUrd +/- FdUrd. These findings suggest that BrdUrd may be superior to IdUrd as a radiation sensitizer in the treatment of colorectal cancer metastatic to the liver.