A method for determining the distribution of lysozyme activity of animal tissues in situ has been devised. This method was based on a microscopically visible change in the staining reaction of Micrococcus lysodeikticus after incubation with cryostat sections of fresh frozen tissue. The sections were superimposed on a specially prepared film of bacteria. The change in the bacteria, apparently proportional to the lysozyme activity of the apposed cells, was shown by staining with a mixture of Alcian blue and basic fuchsin. This also stained the animal tissue suitably for histologic examination. Lysozyme distribution in the normal mouse can be summarized as follows: (1) very active in polymorphonuclear leukocytes, Paneth cells, and hemopoietic elements of bone marrow (2) moderately active in lymphocytes (large more active than small), lung (apparently homogeneous), proximal renal tubule (more than glomerulus), pancreatic acinar cells, stomach glands, and lamina propria of small intestine and colon (3) little or no activity in macrophages (except in lung), megakaryocytes, brain, liver, thymus, thyroid, adrenal, salivary gland, testis, upper epithelium of gastrointestinal tract, nor smooth, skeletal nor cardiac muscle. Changes in lysozyme activity in the irradiated mouse at 9 days may be summarized as follows. There was an increase in lysozyme activity in the proximal tubules and glomeruli of the kidney. Acinar cells of the pancreas lost most of their activity. In the radiation chimeras the polymorphonuclear (PMN) leukocytes showed an increase in individual activity as well as increased numbers over normal. There was increased activity in the lamina propria of the gastrointestinal tract. All other changes noted could probably be attributed to the transplantation and proliferation of foreign cells and redistribution of the cellular elements of the host animals. Acid phosphatase activity was determined for comparison, in the same tissues, with lysozyme distribution. Acid phosphatase activity was high in macrophages, acinar cells of liver and salivary glands, villus epithelium of small intestine, proximal tubules and glomeruli of kidney. These showed apparently constant acid phosphatase activity in the normal and treated animals. Whole organ changes seemed correlated with modified cell distribution.