In previous studies from this laboratory (1, 2) it was found that the ability of mouse spleen homogenates to afford protection against radiation death, when injected into otherwise lethally X-irradiated mice, is associated with the nuclei fraction. Isolation of the spleen nuclei with retention of protective activity in the fraction was accomplished by the use of a fortified sucrose-salt medium in which the subcellular fractions were obtained by differential centrifugation of the spleen homogenate. No protective effect was observed after injection of spleen mitochondria, microsome, or soluble supernatant fractions. Attempts to obtain active nucleic acid extracts from the nuclei fraction, employing the molar NaCl extraction procedure of Mirsky and Pollister (3), yielded negative results. It is recognized that the deoxypentosenucleic acids, in association or combination with protein, comprise the major chemical constituent of the cell nucleus. In view of experimental evidence in the literature (4, 5) indicating that treatment of deoxypentosenucleoprotein complexes with 1 M NaCl elicits profound alterations in their physiochemical properties (e.g., dissociation of protein-nucleic acid linkages, changes in viscosity), it was proposed (2) that the protective factor in mouse spleen may be a nucleoprotein. The experiments to be described here were designed to test the validity of this hypothesis and to obtain further data on the chemical nature of the factor. To this end, crystalline enzymes have been employed as specific analytical reagents in the present study. As is well known, most enzymes are characterized by a marked specificity of action toward their substrate and exhibit activity under physiological conditions; these properties make possible their use in vitro as analytical reagents and provide a unique means of investigating the chemical