Radiation sensitivity was determined for nine University of Michigan squamous cell carcinoma (UM-SCC) cell lines, MCF-7 and HeLa, using a 96-well plate clonogenic assay. Plating efficiencies (PE) of the UM-SCC cell lines were between 0.16 and 0.36. Higher PE values obtained were with MCF-7 (0.4) and HeLa (0.5). The UM-SCC cell lines were used at low passages (passage 13 to passage 20) to minimize artifacts attributable to long-term culture. Cells were irradiated in suspension using a cobalt 60 gamma source at a dose rate of 0.94 Gy/min (94 rad/min). Survival data were fitted well by either a linear quadratic function F = e-(alpha D + beta D2) or by a monoexponential function F = Ae-alpha D. Mean inactivation dose, equivalent to the area under the survival curve (AUC), was used as a measure of radiation sensitivity. The UM-SCC-1, 9, 11A, 11B, MCF-7, and HeLa were the most radiation resistant lines we tested (AUC greater than 2.1), while UM-SCC-14A was the most sensitive (AUC = 1.591). The assay was highly reproducible, and the difference in radiation sensitivity between cell lines were statistically significant.