Our efforts in the coming year will be directed towards a more intensive study of TdT-positive cells. Specifically, we will study the ultrastructural characteristics of TdT-positive bone marrow cells and attempt to determine the intracellular localization of terminal transferase at various phases of the cell cycle. We will study the hemopoietic and lymphopoietic developmental potential of TdT-positive bone marrow cells in adoptive transfer studies, in semi-solid agar cultures, and on monolayers of bone marrow stroma and thymus epithelial cells. We will attempt to use the latter system to grow purified populations of TdT-positive cells for future cloning experiments. As an aid to the isolation and/or suppression of TdT-positive cells, we will attempt to develop a specific antiserum to surface antigens on these cells. We will continue to study the ontogeny of TdT-positive thymocytes in organ cultures with a view towards determining the developmental relationships between these cells and TdT-negative cortical and medullary thymocytes.