5' Amino-5'-deoxythymidine (5'-AdThd) has been demonstrated previously to antagonize dTTP-mediated feedback inhibition of purified thymidine kinase from 647V, a human bladder cancer cell line. Low concentrations of 5'-AdThd (3-30 microM) have also been shown to stimulate cellular uptake of iododeoxyuridine (IdUrd) in 647V cells at clinically relevant IdUrd concentrations (2 microM). We report that the combination of 30 microM 5'-AdThd plus 2 microM IdUrd results in a significant increase of IdUrd replacement of thymidine (dThd) (18%) in the DNA of 647V cells over that obtained by exposure to 2 microM IdUrd alone (7.9%). However, increasing the 5'-AdThd concentration to 300 microM inhibited the incorporation of IdUrd into DNA (3%). IdUrd-induced radiosensitization of 647V cells, as measured by clonogenic survival, was enhanced by coincubation with 30 microM 5'-AdThd, while 300 microM 5'-AdThd reduced the IdUrd radiosensitization. Additionally, radiation-induced single strand break generation when IdUrd was incorporated into 647V DNA, as measured by rapid alkaline elution, was also enhanced by coincubation with 30 microM 5'-AdThd, while 300 microM 5'-AdThd resulted in a decrease in the number of single strand breaks produced. In T24, another bladder cancer cell line, and SV-HUC-TT1, a tumorigenic cell line derived from SV-HUC, 3-10 microM 5'-AdThd was also able to enhance IdUrd replacement of dThd in DNA. However, no stimulation of dThd replacement by 5'-AdThd occurred in SV-HUC, a prototypic "normal" bladder urothelial cell line. Since 5'-AdThd is not a substrate for mammalian thymidine kinase and has little or no cytotoxicity in vitro and in vivo, it may be a selective modulator of IdUrd radiosensitization of human bladder carcinoma and should be tested in vivo.