Background: This investigation is done to evaluate the extent of β-Interferon-induced radiosensitization and the underlying mechanisms by which β-Interferon alters the radiation response of two different human tumours in vitro. Material and Methods: Two human tumors were studied in vitro. One was the second passage of a human squamous cell carcinoma of the head and neck (ZMK-1), the other was the breast cancer cell line MCF-7. Cells were cultured by standard methods. β-Interferon was used in concentrations from 10 to 1,000 IE/ml medium, incubation times were 3 h (pulse) or for the remainder of the incubation for colony formation (continuous). Cells were either attached to their flasks or in suspension. The cells were irradiated with 200-kV X-rays at dose levels from 2 to 10 Gy. Evaluation was done by colony assay. Results: For MCF-7 cells we found an antiproliferative effect caused by β-Interferon depending on concentrations and incubation times. When combined with X-radiation we found no additional effects of β-Interferon for both kinds of cells irradiated in suspension. When the cells were irradiated attached to their flasks, β-Interferon caused a slight radiosensibilization with respect to the MCF-7 cells and a pronounced effect with respect to the ZMK-1 cells, the isoeffect enhancement ratio being 1.5, determined at the 10% survival level. Conclusion: These results demonstrate that under the influence of β-Interferon repair of sublethal radiation damage of MCF-7 and ZMK-1 cells is decreased by a short preirradiation incubation time. Additionally, lethal radiation of ZMK-1 cells is increased by β-Int