The Mark I cesium-137 irradiator is a modern gamma-ray irradiator designed for biological research. It is suitable for the irradiation of small animals, tissue culture, feeder layers, bacteria, etc. in a temperature controlled, humidified or dry atmosphere of air, N2, or other gases. This instrument is needed sorely by at least six principal investigators at Yale who have 13 active PHS peer-reviewed research grants. Their research is generally in the area of cancer research, encompassing wide variety of interests including radiobiological studies of the americium- 241 at low dose rates, enhancement of IUdR radiosensitization by low energy photons, use of perfluorochemical emulsions as adjuctant to radiotherapy, study of intracellular PH and anti-tumor drug efficacy, measurement of tumor hypoxia using NMR imaging and spectroscopy, imaging of 3-dimensional dose distributions using NMR, water calorimeter for radiation dosimetry, hypoxic cell selective chemotherapeutic agents as adjunct to radiotherapy, influence of H-2 antigens on T-cell responses, immunoregulation of T and B lymphocytes. All of these research projects need a reliable, modern gamma irradiation facility which offers the versatile options of a wide range of dose rates, collimation and controlled environment for irradiation samples. However, a modern gamma irradiator is too expensive to acquire by individual investigators. Therefore, we propose to develop a shared facility for gamma irradiation of biological materials. It would greatly enhance the quality of PHS peer-reviewed research activity at Yale, and would also serve as a research resource to the wider research community at Yale.