Measurements were made on the absorption, in concrete, of the primary and secondary radiation from a 4 MV linear accelerator. The results give tenth-value layers for primary and secondary radiation of 12 inches and 9 inches respectively, in concrete of density 2.2 g/cc. The distribution of tube leakage radiation, and of secondary radiation scattered from a phantom in the beam were also examined. Scattered radiation is only significant in the forward direction.