Mouse leukemic cells (L5178Y), Chinese hamster lung cells (V79- S171), and SV-40 virus transformed hamster cells will be labelled with tritiated precursors of DNA and/or RNA after these cells have been synchronized by selective mitotic cell detachment or by blocking with hydroxurea. These cells will then be frozen to -196 degrees centrigrade and damage induced by tritium disintegrations. After various periods of time the cells will be thawed out and the reproductive ability of the cells measured by standard methods. The efficiency of cellular inactivation will be compared for damage accumulated in various subcellular regions. These experiments are designed to determine possible regions within the mammalian cell which when damaged lead to reproductive death. Initial regions that may be compared using this method are the nucleolus, nucleus, and early and late DNA replicating regions including parts of sex chromosomes and mouse satellite DNA. The extent of damage of cells and degree of damage correction at different ages in the cell cycle can also be compared by labelling the same subcellular region and freezing the cells at different cell cycle times. The quality of the damage at particular subcellular sites will also be varied by using isotopes other than tritium like I125 in the DNA precursor IUdR. The results of these studies may provide information of two general types 1) information on the basic mechanisms which lead to the inactivation of proliferative ability of mammalian cells from normal, oncogenically transformed, and cancerous cells in vitro and, 2) quantitative estimates of some of the possible hazards from environmental tritium which will be released into the environment as a by-product of nuclear power reactors in the form of tritiated water.