Iododeoxyuridine is a halogenated pyrimidine and non-hypoxic cell radiosensitizer currently being used in clinical trials. The amount of radiosensitization by IdUrd is related to the amount of incorporation of the drug into a cell's DNA. These experiments were carried out in three human tumor cell lines (lung, glioma, and melanoma) in monolayer culture exposed to concentrations of IdUrd from 0.1-10 microM for one and three cell cycles before irradiation to determine incorporation and sensitization as a function of drug exposure. Except for the lung cell line, which required greater than 1 microM IdUrd, these cells demonstrate radiosensitization when exposed to 0.1 microM or greater of IdUrd. Maximum sensitization occurred at 10 microM IdUrd for all the cell lines at three cell cycles. The percent thymidine replacement by IdUrd increased with increasing concentrations, but was cell line dependent. Maximum percent replacement occurred at 10 microM at three cell cycles for all the cell lines: lung = 22.4%, glioma = 32.0%, and melanoma = 39.1%. The relationships between percent thymidine replacement and sensitization are not identical across these human tumor cell lines. If IdUrd is going to be a successful radiosensitizer in clinical trials, sustained plasma levels of 10 microM or greater for at least three cell cycles should be achieved during irradiation. This may be best accomplished with repeated short exposures to IdUrd (three cell cycles or approximately 4 days in these cell lines) every 1-2 weeks during radiation. Measurements of thymidine replacement in a tumor biopsy should be attempted prior to radiation to develop a predictive assay for radiosensitization.