||1. Total body x-irradiation of mice in doses of 300, 400, or 500 r does not affect the phagocytosis by reticuloendothelial cells of spleen of intravenously injected ThO2 when determined 2 day following x-irradiation. Similar studies carried out on the seventh post-irradiation day revealed (1) that 300 r of whole body x-irradiation had no influence on the 2, 6, and 24 hr splenic uptake values; (2) that 400 r of whole body x-irradiation caused a transient depression of splenic uptake of ThO2 at 2 hours but 6 and 24 hours values were not different from the controls and (3) that 500 r of whole body x-radiation significantly depressed the uptake by the spleen of ThO2 at 2, 6, and 24 hours after injection when compared to the controls. These observations indicated that the effect of x-irradiation on phagocytosis by reticulo-endothelial cells is a function of x-irradiation dose and the post-irradiation period of observation that phagocyte function was measured. 2. A dose of 550 r (LD50) significantly depressed the phagocytosis of ThO2 by reticulo-endothelial cells of spleen in adult, LAf1 mice where as a comparatively high dose, 500 r (LD99) is required to suppress the phagocytic function in adult albino mice. 3. Chronic exposure of mice to x-irradiation in dose of 25 r at intervals of 2 days, until accumulated doses reached as high as 600 to 825 r, failed to yield alterations in the phagocytic activity of reticulo-endothelial cells or the cytopeptic activity of peritoneal phagocytes. It is possible that the resultant injury following successive exposures to ionizing radiation is a function of the intensity of the radiation dose, the interval between the previous and the following dose, and the duration of such treatment. 4. X-irradiation is doses of 350 or 450 r in mice significantly depressed the intracellular digestion of chicken erythrocytes by peritoneal phagocytes when studied on the sixth post-irradiation day. Similar observations were obtained when rabbits were exposed to 600 r of x-irradiation and cytopeptic activity was determined on the seventh post-irradiation day. 5. Immunization with chicken erythrocytes caused an increase in the digestion of these cells by the phagocytes of mice and rabbits. This immunization induced increase in digestion was completely reversed by x-irradiation although x-irradiation did not appreciably alter the hemolysin titers of the immunized animals. 6. In vitro experiments concerning phagocyte digestive activity carried out in the presence of serum from immunized and immunized-irradiated rabbits showed no difference in digestive activity. Furthermore the finding that the phagocytes from immunized-irradiated animals did not digest chicken erythrocytes any more efficiently than phagocytes from non-immunized x-irradiated mice even though the hemolysins titers were comparable in the immunized and immunized-irradiated animals suggest that the salutary effect of immunization and the detrimental effect of x-irradiation on phagocytes themselves and not dependent on antibody which appeared following immunization. 7. Piromen, a polysaccharide extracted from Pseudomonas aeuroginosa, in doses of 1 ug per injection given subcutaneously in mice twice daily for 3, 5 or 7 days, significantly increased the cytopeptic activity of peritoneal phagocytes in mice. Similar treatment employing a dose of 0.1 ug also increased the cytopeptic activity when animals were treated for 5 or 7 days but no increase in intracellular digestion was noted when treatment was carried out for only 3 days. 8. Studies made of the resistance of normal mice injected intraperitoneally with phagocytes from mice immunized with K. pneumoniae vaccine to an intraperitoneal challenge with the homologous organism indicated that, although there is no significant difference in their overall mortalities, mice treated with leucocytes from immunized mice had a mean survival time slightly longer than the normal mice.