Thymocyte subsets have been well characterized on the basis of CD4 and CD8 antigen expression. Recently, the use of anti-CD3 antibodies has allowed more precise phenotyping of these subsets. The most Immature T cell precursors are largely CD3−CD4−CD8−, while the most mature are CD3brightCD4+CD8− or CD3bright;CD4−CD8+. Moreover, the expression of CD45RA on thymocytes appears to define a progenitor population and may define a continuous lineage of cells. Using a panel of CD45RA antibodies, we have further characterized the CD45RA+ thymocyte population in the murine system. The size of this subset is greatly enhanced in cortisone-treated mice and in sublethally Irradiated mice. Moreover, the CD45RA+ population is present early in foetal life and is maintained thereafter. Using three-colour immunofluorescence, we show that (i) while most CD45RA+ cells are present amongst the CD4−CD8− thymocyte subset in the normal thymus, after cortisone treatment or irradiation, all four thymocyte subsets co-express significant amounts of CD45RA. This suggests that not only progenitor cells but also the mature population which can survive such manipulation are CD45RA+and (II) a large proportion of CD45RA+ cells are CD3bright and this subset is represented in the thymus at all stages of maturation tested. These data suggest that a proportion of TCR-γδ+CD3+ cells in the fetus as well as of TCR-αβ+CD3+ cells in the adult co-express CD45RA.