This Shared Resource began in 1991, with funding provided by the Department of Therapeutic Radiology. It major equipment is a Cesium-137 laboratory irradiator (Mark I, Model 68A) manufactured by J.L. Shepherd an< Associates, Inc. It contains two cesium-137 sources with activities of 6,000 and 100 curies each. This instrument has been purchased by Yale University Medical School with funds provided by a Share< Instrumentation Grant from the National Institutes of Health (grant number 1-S10-RR04813-01;Principe Investigator, Dr. Ravinder Nath). The Cesium-137 Irradiator Shared Resource (CISR) provides access to irradiations by ionizing radiation, specifically 662 keV gamma rays from the radioactive decay of cesium-137. This facility can operate at dose rates ranging from 0.01 cGy/min to 1500 cGy/min. The purpose of the Cesium-137 Irradiator is to provide an accurate and reliable irradiation resource for molecular compounds, cell culture, animal and human tumor cells, and whole animal experiments. Our specific aim is to keep an irradiator available 24 hrs/day, 365 days/year for any clinical or research protocol that requires gamma irradiation. This cancer-center core facility provides various scientists in the Cancer Center the ability to study the effects of specific radiation-induced lesions in the DNA of cells, as well as what repair pathways are involved after radiation damage, and other radiobiological mechanisms. The radiation delivered also can be used as a tool in cancer research to suppress the immune response in cell cultures or animals by generating irradiation-induced bone marrow chimeric mice. The CISR has been instrumental in the development of 3-dimensional polymer based dosimeters for applications in radiology, as well as the investigations of the radiation physics and the dosimetric response of superheated emulsion gels. In hematology labs, gamma irradiation of cellular blood components is still the acceptable method for the prevention of transfusion-associated graft-versus-host disease. This CISR can also be used for the sterilization of various cell, vaccine and drug preparations. Since its inception, the CISR has enabled 104 principal investigators (76 YCC Members) from 38 departments/sections at Yale the ability to perform various cancer research projects involving irradiations over a wide range of dose rates. In the past year, 43 YCC members, from 7 out 8 of the YCC Research Programs, generated 92% of the total usage (85% peer-reviewed) of the CISR.