THE subject of protective walls is of interest to all who are planning new or revamping old laboratories. Investigation shows that the ordinary tile-and-plaster wall so widely used in building construction is not adequate protection against X-rays produced at radiographic or therapeutic voltages, when the tube is in an open lead glass bowl, in the size of room that is ordinarily allotted to X-ray purposes. It is true that such walls can be covered with lead to give any desired protection, and there is no better protection than this method can afford, provided the lead has a sufficient thickness and is properly laid. In fact, any other wall materials should be compared to lead as a standard. The use of barium plaster has been advocated as an X-ray protective wall material (1) (2) (3). Available information regarding the value of barium plaster walls is conflicting. Inasmuch as we were preparing to build a new laboratory it was decided to make some comparative tests as to the relative value of barium walls and lead walls. Several samples of barium plaster walls were made and tested. Only one type proved successful, this plaster being made of one part barium sulphate, one part Portland cement, and two parts of barium sulphate sand (crushed barium rock). Photographic tests were made of sections of wall of varying thickness using the barium plaster described above. The tests were made as follows: a section of wall and a lead echelon with uniform steps were laid side by side on the same film (film enclosed in Eastman cardboard holder) and exposed to the X-rays. In this way the tests were independent of exposure times, fluctuating milliamperage or voltage, photographic emulsion, developer temperatures, etc. The results of the tests were as follows: less than ½ inch of barium plaster fails to act as a homogeneous filter and cannot be said to have a lead equivalent;¾ inch barium plaster is equivalent at radiographic voltages (8 inch gap) to approximately 1∕16th inch metallic lead; at therapeutic voltages (200 kilovolts) 2 inches of barium plaster, weighing 30 pounds to the square foot, is equivalent to approximately 5∕32ds inch of metallic lead. The protective coefficient, the ratio of the lead equivalent thickness to the thickness of the barium wall, is in the first case 0.083 and in the second case 0.078, apparently constant for the two voltages. Hunt (2), for a somewhat similar type of plaster, reports protective coefficients for these voltages of 0.126 and 0.09, respectively. Dr. L. R. Sante (3) has reported on this type of wall, on the basis of ionization tests, as follows: 2½ inches of plaster is equivalent at 200 kilovolts to approximately ¼ inch lead, protective coefficient 0.1. Neither Sante nor Hunt gives the weight per square foot of the samples reported.