The combined effects of fractionated administration of the antiviral agent (E)-5-(2-bromovinyl)-2'-deoxyuridine (BVDU) and fractionated (4 injections at a 48-h interval) administration or continuous (7-day) infusion of 5-fluorouracil (FU) have been investigated in P388 leukemia-bearing mice. The LD50 of continuous FU infusion in normal mice was approximately 330 mg/kg, while that for the fractionated treatment was 160 mg/kg. When combined with BVDU (200 mumol/kg, 90 min before each FU injection; 4 injections at a 48-h interval), the LD50 of FU was decreased from 330 to 115 mg/kg and from 160 to 30 mg/kg, respectively. The 7-day continuous infusion of FU (up to a dose of 210 mg/kg) did not exert a therapeutic effect in P388-leukemic mice, and, similarly, combination of continuous FU infusion (at lower doses) with BVDU failed to increase the survival of the leukemic mice. If, however, FU was administered in fractionated doses, there was a dose-dependent increase in survival time of P388-leukemic mice, and simultaneous fractionated BVDU treatment allowed a 4-6-fold lower dose of FU to achieve the same increase in survival time, indicating that BVDU effectively increased the antitumor activity of FU in the leukemic mice.