During the past five years, major developments have been made in flow systems for rapid cell analysis. The Biophysics and Instrumentation Group of the Los Alamos Scientific Laboratory is one of a few groups that pioneered this instrumentation development. This instrumentation has potential applications in biology and medicine. The objective of this proposal is to apply this instrumentation to some of the selected problems in radiobiology related to radiotherapy. Preliminary investigations indicate that these flow systems could be used to study cell viability, DNA distribution of cells, and changes in cell-cycle distributions after exposure to agents such as ionizing radiation and chemotherapeutic drugs. This instrumentation could also be useful in tumors with elevated DNA content to monitor quantitative changes in the distribution of tumor and normal cells in tumor biopsies during the course of radiotherapy. The proposed research is aimed to explore the potential application of this instrumentation to cell viability studies, radiobiology studies in rodent tumors and multi-cellular spheroids, and DNA measurements of spontaneous tumors in dogs and human tumor biopsies (carcinoma of the cervix) to see if there is a correlation between DNA content and radiocurability and the possibilities of monitoring the efficacy of treatment during the course of radiotherapy.