The potential of zalcitabine (ddC) to act as an ionizing radiation response modifier was tested on exponentially growing human cancer cells in vitro. Two human cell lines, WiDr (colon) and MCF-7 (breast) were exposed to ddC at 10 microM concentration for various lengths of time (18, 24, 48 and 72 h). On the WiDr cell line the dual effect of concentration and duration of exposure prior to irradiation was investigated. Experimental endpoints were clonogenicity and viability, as measured by colony formation assay (CFA) and MTT assay respectively. The impact on cell-cycle distribution prior to irradiation was assessed by flow cytometry using a double labeling technique (propidium iodide and bromodeoxyuridine pulse label). A significant reduction in surviving fraction and viability was observed for WiDr-cells irradiated after pre-exposure to 10 microM for 18, 48 and 72 h as compared to corresponding irradiated controls. At lower concentrations (1 and 5 microM), the radiosensitizing effect was only significant after a 72-h exposure (assessed by CFA). For MCF-7, ddC induced a significant modification of the dose response only with 24 and 48 h preincubation. However, the overall effect was less pronounced as compared to WiDr. Cell-cycle analysis showed accumulation in S-phase, 48 and 72 h after treatment with 10 microM ddC in the WiDr cells, with a progressive shift to late S-phase as shown by the biparametric analysis. The degree of radiosensitization is cell-line dependent with the most important sensitization observed on the most "radioresistant cell line", i.e., the cell line with the lowest alpha value and highest SF 2 (WiDr). For WiDr, radiosensitization by ddC depends on the duration of exposure and the concentration of the drug.