The assumtion is often made in x-ray protection planning that conventional building materials provide no shielding. The exception is concrete which, due to wide use, is accepted by everyone as a satisfactory shield. It is not at all unusual to encounter instances where any material other than lead or concrete is considered unacceptable for wall construction without the addition of a very thin sheet of lead. All materials provide some protection, however, and it would seem advisable to take this fact into consideration in shielding calculations. Some types of construction involve materials of a heterogeneous nature and the problem of calculating shielding data under such circumstances does not seem to be a simple one. The only solution apparent to us was actually to construct some typical wall sections and make transmission measurements. The x-ray source used in these investigations consisted of a resonant transformer operating at a nominal frequency of 1,200 cycles, and a grounded anode, beryllium window tube, capable of operation from 70 to 300 kvp at tube currents up to 20 ma. An electronic power source supplied the 1,200-cycle input to the high-voltage transformer. Both kilovoltage and tube current were electronically stabilized and monitored with 0.1 per cent potentiometers having 0.05 per cent reproducibility. Radiation measurements were taken at 70 to 300 kvp, with 0.5 mm. of aluminum filtration up to and including 125 kvp and with 3.0 mm. of aluminum at higher kilovoltages. The filtration was selected to be consistent with that used in connection with the lead attenuation curves in National Bureau of Standards Handbook 60, Fig. 3, page 35. The x-ray unit was provided with a diaphragm which could be adjusted for square or rectangular fields. This diaphragm with its associated light-beam localizer was used to set the field at the front surface of the wall sections. Transmission measurements were made with the smallest practicable ionization chambers better to resolve small holes in the wall sections. All dose rate measurements greater than 1 milliroentgen per minute were taken with a Victoreen 25-r thimble chamber modified and connected as shown in Figure 1. For a few of the measurements at low kilovoltage, a 100-c.c. chamber was used. Comparison of the two chambers showed good agreement, both behind the wall sections and in the unattenuated beam. Ionization currents were read with a null system with a 0.1 per cent potentiometer and a d.c. amplifier as a null detector. The system was calibrated against a Victoreen r meter at dose rates sufficiently high to allow intercomparison. The Victoreen r meter had been calibrated at the National Bureau of Standards. In order to provide data on structures conforming as nearly as possible to that which would be found in actual construction, several trade organizations were asked to provide samples of what they considered to be typical walls.