An experiment is presented in which the roentgenographic properties of the chemically inert gas, xenon, were studied in a phantom whose proportions were designed to simulate a human chest. The experiment was performed utilizing roentgen rays of 30, 60, and 100 peak kilovoltage, and varying amounts of added aluminum filtration, ranging from 0.5 to 2.0 mm. A system of densitometry was used which included the pre-exposure of a portion of all roentgenograms with a stepped wedge in place, to be used as a frame of reference and to determine film response. The results of the experiment indicated that (1) xenon, in path-lengths ranging from 1.04 to 9.80 cm., does cast a faint roentgenographic shadow relative to air, and that differences of radiopacification can be appreciated visually as well as detected densitometrically; (2) the difference of radiopacification of xenon relative to air, as determined densitometrically, is accentuated in exposures made at 30 and 60 kvp relative to those made at 100 kvp, but no difference can be detected between the exposures made at 30 kvp and those made at 60 kvp; and (3) the addition of 0.5 to 2.0 mm. of aluminum filtration had no effect on the relative radio-pacification of xenon and air at 60 to 100 kvp, but diminishes this radio-pacification at 30 kvp. The faintness of the roentgenographic shadow cast by xenon leaves some doubt as to the clinical usefulness of this procedure with ordinary roentgenographic techniques. A preliminary clinical study is presented to demonstrate this. The physical and physiologic properties of xenon are presented, especially with regard to the general anesthetic properties of the gas.