The UCHL1 monoclonal antibody specifically reacts with human CD45RO, a 180 kDa isoform of the leukocyte common antigen CD45. CD45RO is a transmembrane glycoprotein with tyrosine phosphatase activity and is expressed by majority of thymocytes, monocytes, granulocytes, and activated memory T lymphocytes. The subsets of peripheral T lymphocytes can be discriminated by using the CD45RO and CD45RA expressing cells.