Selective exposure of local areas to roentgen radiation is usually simple to accomplish by blocking off all but the exposed area with sheets of lead. If the volume exposed is relatively small, the radiation which is scattered into the shielded regions is apt to be negligible. On the other hand, selective shielding of local areas may be a difficult problem, since in this case the scattering volume is often relatively large. Because of the frequent need for selective shielding in biological work, a study has been made of the contribution of scatter from remote regions to the dose delivered at a particular point in a phantom. The experimental arrangement is shown in Figure 1. A circular lead shield was mounted atop a Presdwood phantom co-axially with the vertical x-ray beam. By means of a Victoreen thimble ionization chamber, radiation intensities were measured in the center of the beam at two depths within the phantom, namely, 1.6 cm. and 13.6 cm. The phantom was 23.3 cm. deep overall (including a 2.2 cm.- ...