We propose to utilize a new detector material, polycrystalline mercuric iodide, for background suppression by active anticoincidence shielding in gamma-ray spectrometers. Two proposed NASA missions will require anticoincidence shielding for gamma-ray spectrometers: the Mars Lander and a Space Science Vision Mission expected to visit Titan, one of Saturn?s moons. Shielding improves the performance of gamma-ray spectrometers by reducing the effect of charged particle interactions which can not be distinguished from true gamma-ray interactions by the spectrometer. Active shields produce a blanking signal when a charged particle is detected, so that the signal from the spectrometer can be ignored during the spectrometer?s charged-particle interaction. While it is well know that this technique produces significant improvement in gamma-ray spectrometer performance, the technology to implement it is lacking. The attributes of mercuric iodide make it an excellent candidate for anticoincidence shielding detectors. Because of its detection characteristics, light weight, small size, low cost, robustness, and ease of application to non-planar geometries, this material can replace the costly, heavy, and bulky scintillator/photomultiplier tube (PMT) systems currently in use. The application of this new material for space-based astrophysical observations will provide excellent background suppression with improved mass and volume characteristics.