With the increasing number of high-energy linear accelerators being installed in radiation-therapy facilities, an increasing need exists for carefully measured data concerning the shielding parameters of photon radiation in the energy range above 10 MeV. In this study, the 18 MV X-ray beam from a Varian Clinac® 20 linear accelerator was employed. Transmission parameters were measured for the primary X-ray beam in concrete and steel. The tenth value layer for steel was 11.3 ± 0.6 cm and for concrete 45.0 ± 2.0 cm. Also measured were the transmission parameters for scattered radiation vs angle in concrete and steel.