Technical Abstract: Food irradiation is a safe and effective method for inactivation of pathogenic bacteria including Escherichia coli O157:H7 in meat, leafy greens, and complex ready-to-eat foods without affecting food product quality. Determining the radiation dose needed to inactivate E. coli O157:H7 in foods, and the validation of new irradiation technologies is often performed through inoculation of model systems or food products with cocktails of the target bacterium, or use of single well-characterized isolates. In this study the radiation resistance of 4 E. coli strains, two of which are DNA repair deficient strains used for cloning and recombinant DNA technology (JM109 and DH5alpha), and two strains of serotype E. coli O157:H7 C9490 and ATCC 35150 were determined. The D-10 values for C9490, ATCC 35150, JM109, and DH5alpha stationary phase cells suspended in Butterfields Phosphate Buffer and irradiated at 4C were found to be 0.229 (9.00), 257 (7.00), 61.2 (10.4), and 51.2 (8.82) Gy, respectively. Use of E. coli JM109 and DH5alpha which carry mutations of the RecA and GyrA genes required for efficient DNA repair and replication, is not appropriate for determination of radiation inactivation kinetics and validation of radiation processing equipment.