Recognizes a 180-185kDa protein, identified as isoform of leukocyte common antigen (CD45RO). This antibody reacts with mature activated T-cells, most thymocytes, and a sub-population of resting T-cells within both CD4 and CD8 subsets. It shows no reactivity with normal B or natural killer cells, but reacts with granulocytes and monocytes. Reportedly, it is useful to identify T-cell lymphomas and leukemias. It rarely stains NK cells or B-cell lymphomas.