AbstractThe thymic microenvironment involves complex cell interactions among different types of epithelial cells, macrophages, tissue histiocytes and immature and maturing T cells. We describe the isolation of a subset of thymic epithelial cells by selective primary culture followed by SV 40 ori‐and psV‐2 neo co‐transfection. The cloned cells were characterized as subcapsular cortex epithelial cells having hemopoietic growth factor activity, CSF‐1 and LFA3 associated binding sites for T cells. Thus cotransfection with the selectable genes and the gene which gives growth advantage to the transformant enabled us to investigate the rare type of cells which has been difficult to isolate by ordinary tissue culture technique because of mixed cellularity and fibloblast ove