Chicken CD45 is a transmembrane glycoprotein expressed on all leukocytes. B lineage cells in the bursa of Fabricius express the 215kD isoform while T lineage cells bear the 190kD variant. These high molecular weight molecules have intrinsic phosphotyrosine phosphatase activity characteristic of mammalian CD45.1 Levels of CD45 expression detected by this MAb in the avian thymus are heterogeneous with approximately 90% of thymocytes expressing fourfold higher levels of surface CD45 (CD45hi) than do the remaining 10% (CD45lo) of thymocytes. The CD45lo population contains exclusively thymocytes with the CD3-CD4-CD8lo phenotype characteristic of the immediate precursors to the CD3-CD4+CD8+ thymic population which are CD45hi. This shift from low to high levels of surface CD45 expression is concomitant with the transition from CD4-CD8lo to CD4+CD8+ and before the expression of CD3. The ability of this antibody to discriminate this CD45lo subpopulation provides a suitable marker for the CD3-CD4-CD8lo immediate precursors to the CD3-CD4+CD8+ thymocytes.1