The HI30 monoclonal antibody specifically reacts with the 180 kDa, 190 kDa, 205 kDa, and 220 kDa isoforms of the human leukocyte common antigen (LCA) CD45. It is expressed on lymphocytes, granulocytes, monocytes, thymocytes, and eosinophils, but not on mature erythrocytes, platelets, mature erythroid cells of bone marrow, and non-hematopoietic tissues. CD45 is essential for T cell activation and the tyrosine phosphatase activity of its intracellular region is integral for signal transduction.