The role of thymus gland and specifically of the thymic epithelium in the maturation and differentiation of incompetent T precursors into immunocompetent and H-2 restricted T lymphocytes is well established, The thymic nurse cell (TNC), the most recently identified thymic epithelial cell containing engulfed thymocytes is considered to be the potential site for lymphocyte maturation, education and differentiation in the thymus. The study of the development of TNC; its morpholog: surface phenotype:; electronmicroscopic observation of the intr-TNC environment: confirmation of its epithelial nature using a monoclonal antibody to keratin: nature of TNCs obtained from thymuses of mice either irradiated, treated with hydrocortisone , in vitro cultured or implanted in the peritoneal cavity in a millipore chamber: standardization of proliferation and cytotoxic assays in a microculture using hanging drops in Terasaki plates: surface phenotype and functional characteristics of T cells derived from TNCs, will form some of the topics of the present thesis. The study confirms the epithelial nature of TNCs by immunofluorescent staining of keratin filaments in them. The study also suggests that TNCs may represent a subset of epithelial cells of cortical origin which provide a suitable microenvironment for the development of a subpopulation of cortical thymocytes which in turn acquire some stage of functional maturity when studied in vitro. The thesis will discuss in detail the involvement if any of TNCs in the T cell maturation process in the thymus.