We have previously reported a monoclonal antibody (mAb), U5A2-13 mAb, which originally recognizes a phenotypically and functionally similar population of natural killer (NK)-like T cells. In this study, we found that U5A2-13 antigen (U5A2-13) was expressed not only on NK-like T cells but also on T and B cells during activation. In contrast to the low levels of U5A2-13 on freshly harvested T and B cells, the activation of these cells by various stimuli resulted in high levels of expression of U5A2-13 in vitro and in vivo. Similar to CD69, U5A2-13 is also expressed in most mouse lymphoid cell lines but not in nonhematopoietic cells. U5A2-13 on T cells reached maximal expression by 24h after stimulation and returned to baseline levels after 3 days. However, U5A2-13 differed from CD69 since its expression profile was different on CD4(+)- and CD8(+)-activated T cells, phorbol ester-activated EL-4 cells, and activated splenocytes in CD69-deficient mice. In addition, immunoprecipitation study indicated that U5A2-13 is not identical to CD69. Importantly, the U5A2-13-positive population of CD4(+) T cells exhibited significant levels of cytokine producing activity upon stimulation. Overall, U5A2-13 is an early inducible cell surface antigen that could be involved in lymphocyte activation.