Studies of cell kinetics in patients with acute leukemia and other forms of cancer in which tumor cells are available for sampling are being done. Proliferative activity of the tumor cell population is being determined by measuring the mitotic index and 3H-Tdr labeling index. Studies of the distribution of cellular DNA content in the tumor cell population are proposed by means of flow microfluorimetry. Particular emphasis will be given studies designed to assess the effect of chemotherapeutic perturbation of the malignant and normal cell populations in order to obtain information useful in designing chemotherapeutic regimens. Attempts will be made to establish methods for cell cultures of acute leukemic lymphoblasts to provide for the opportunity of studying growth regulatory mechanisms of these cells.