It is the purpose of this paper to give the results of depth dose measurements on a 400-kv. machine. Because of the difficulty in making measurements in water, with a removable chamber, the depth doses were measured in a Masonite pressdwood phantom. Pressdwood is not “standard” and may vary in density and composition. It is therefore advisable to compare the particular sample with water before beginning a series of measurements. A comparison was made between the depth doses in our sample of pressdwood and water, using the same ionization chamber in each, at 100 kv., hvl. 1.86 mm. Al, 200 kv., and 400 kv. The depth doses agreed to within about 5 per cent. Since this obtains, this pressdwood phantom can be used for depth dose measurements instead of water at the above voltages. Experimental Work Measurements were made with a Victoreen condenser dosimeter, removable type ionization chamber. The phantom was made of Masonite pressdwood, each board having a thickness of 0.373 cm. and measuring 30 × 30 cm. The density of the material was about 1.05 gm./c.c. The phantom was always 25 cm. thick. A group of six boards was drilled so that the ionization chamber fitted snugly into them. These six boards were divided into two sets. In one set the chamber rested; the other set covered the chamber. These two sets will be called the chamber boards. The surface measurements were made with the ionization chamber half “submerged” in one set of chamber boards in the same manner as surface measurements are made for water. At other depths, measurements were made with the chamber snugly encased in the chamber boards at definite distances from the surface. Measurements were reproducible. The water phantom used for comparisons had the same dimensions as the pressdwood phantom. The radiation from the 400-kv. machine had a half-value layer of 4 mm. Cu when a 2 mm. Cu + 1 mm. Al filter was added. The size of the field was obtained by means of an Oddman light box, which gave the outline and center of the field and also indicated the distance. The adjustable diaphragm at the bottom of the box was located at 53 cm. from the target. The roentgen rays from the 200-kv. machine had a half-value layer of 2.2 mm. Cu. The fields were outlined by means of cones coming down to the surface of the phantom. The limiting diaphragm was 26 cm. from the target on top of the cone. Results for Pressdwood The depth doses obtained by using the Masonite pressdwood phantom, density 1.05 gm./c.c., are plotted for 400 kv. and 60, 70, and 80 cm. F.S.D. for fields of different sizes in Figures 1, 2, and 3. Since it is sometimes more convenient to use tables, Table I has been compiled on the basis of the curves. Comparison of 400-kv. and 200-kv. X-Rays in Pressdwood: The depth doses for 200 kv. were measured for fields 4 × 5 cm. and 10 × 10 cm., at 80 cm. F.S.D. These curves are plotted in Figure 3, which shows, also the doses with 400 kv. for the same distance. The results are tabulated in Table II.