The CD45RA and CD45RO isoforms of the leukocyte common antigen identify functionally distinct naive and memory T cell subsets. In vitro, CD45RA+ / CD45RO - peripheral blood lymphocytes (PBL) can be converted to CD45RA- / CD45RO + phenotype upon activation in the presence of IL-2. Both CD3+ and CD3-/CD56+ lymphocyte subsets can be converted to CD45RO + lymphocytes. Expression of CD45RO was observed only in response to IL-2 and was not observed during long-term culture in IL-4, IL-6, or IL-7. This antibody recognizes an 180kDa 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. Shows no reactivity with normal B or natural killer cells, but reacts with granulocytes and monocytes. Though this antibody is useful to identify T-cell lymphomas and leukemia, rare staining with B cell lymphomas reported.