Several subpopulations of thymocytes have been identified and isolated from the mouse thymus. One of these subpopulations, designated dLy1, has been demonstrated by differentiation studies in vitro and in vivo to represent a very early stage in intrathymic development. These studies were supported by molecular analysis demonstrating that these cells expressed mRNA specific for gamma and beta but not Alpha chain of the T cell antigen receptor. Heterogeneity for beta-chain rearrangements also supported this concept. The current studies have investigated the heterogeneity observed in dLy1 thymocytes by size, surface antigen expression, and beta-chain gene rearrangement. Preliminary results indicate that expression of the IL-2 receptor may correlate more closely to maturation state rather than lineage commitment. A collaborative project has demonstrated that cortical-type thymocytes are the precursors to the L3T4+, Lyt2- medullary-type thymocytes. In vivo treatment with antibodies to Ia, dendritic cells, for Lyt2 prevents the development of specific subsets of T cells. Thus several approaches have been of use and aided in elucidating the T cell developmental sequence and lineage relationships.