In the search for a suitable inethod of determining quantitatively the effects on niamrrialiaii tissue of sublethal doses of ionizing radiation, the following study of the mitotic indices of various mouse tissues following whole body irradiation was undertaken. I t was hoped that by finding quantitative changes in the mitotic index of various tissues that it would be possible to use these clianges as a measure of the relative effects of different types and different energies of ionizing radiation. The response of mitotic activity of 4 tissues was determined at intervals after exposures of groups of animals to doses ranging from 5 to 325 roentgens of 250KV x-rays. The adrenal gland, lymph node, jejuiium and epidermis were selected for this study because of their normally high mitotic activity and because of the simplicity of their structure which lends itself t o the development of qnantitativP methods of counting mitoses. The effect on the epidermis at the 35 roentgen and 32.5 roentgen dosage levels has been previously reported (Knowlton, Hempelrnann and Hoffman, '48) and is presented again to give continuity to the results reported in this article. This document is based on worh performed under Contiart no. 7405-cng-36 fm the Atomic Energy Commission. and Julie Wellnitz. 'With the technical assistance of Pu'orma Lanter, Clnie Norrisnn. Joan Thrxr