This research is designed to achieve four objectives: 1) To study mechanisms and colony stimulating factor (CSA) release from macrophages and lymphocytes. 2) To develop an assay for a thymocyte progenitor cell. 3) To understand the role of totipotent stem cells (CFU-s) in tissues other than bone marrow spleen and fetal liver and 4) To study the effect of murine leukemia viruses on stem cell proliferation in long-term marrow cultures. The methods used will be in vitro development of CSA by incubating macrophages and lymphocytes for varying periods of time and looking at the effect of certain substances and cell viability on this process. Thymocyte progenitors will be studied by plating thymus and bone marrow cells in semi-solid medium and using thymic humoral factor from various sources as a growth promoting substance. Totipotent stem cells from liver will be studied by the in vitro spleen colony assay and their protective effect for lethally irradiated mice. Long term marrow cultures will be carried out using feeder layers of marrow stromal cells.