We investigated radiosensitization in an untreated basal cell carcinoma (TE.354.T) cell line and post-pretreatment with tetraiodothyroacetic acid (tetrac) X 1 h at 37°C, 0.2 and 2.0 µM tetrac. Radioresistant TE.354.T cells were grown in modified medium containing fibroblast growth factor-2, stem cell factor-1 and a reduced calcium level. We also added reproductively inactivated (30 Gy) "feeder cells" to the medium. The in vitro doubling time was 34.1 h, and the colony forming efficiency was 5.09 percent. These results were therefore suitable for clonogenic radiation survival assessment. The 250 kVp X-ray survival curve of control TE.354.T cells showed linear-quadratic survival parameters of αX-ray = 0.201 Gy-1 and βX-ray = 0.125 Gy-2. Tetrac concentrations of either 0.2 or 2.0 µM produced αX-ray and βX-ray parameters of 2.010 and 0.282 Gy-1 and 2.050 and 0.837 Gy-2, respectively. The surviving fraction at 2 Gy (SF2) for control cells was 0.581, while values for 0.2 and 2.0 µM tetrac were 0.281 and 0.024. The SF2 data show that tetrac concentrations of 0.2 and 2.0 µM sensitize otherwise radioresistant TE.354.T cells by factors of 2.1 and 24.0, respectively. Thus, radioresistant basal cell carcinoma cells may be radiosensitized pharmacologically by exposure to tetrac.