Low dose-rate pre-irradiation can make cells more resistant to low doses of acute irradiation. I have found that this effect is due to a mechanism that involves transforming growth factor beta3 (TGF-β3), and have found evidence to how it is induced by radiation. However, it has turned out to be extremely difficult to actually measure TGF-β3. In addition, it is not clear how it works in changing the radiosensitivity of the cells. This presentation will show our latest findings and hopefully lead to a burst of new ideas for interpretation of the data and further experiments from the audience.