CD2–CD48 interactions enhance T cell receptor-driven mouse T lymphocyte activation. However, the mechanism is not well understood. Here we show that blockade of CD2–CD48 interactions with anti-CD48 monoclonal antibody (mAb) inhibited interleukin (IL)-2 and interferon (IFN)-γ expression, as well as T cell proliferation in response to mitogenic anti-CD3 mAb, although more potent inhibition resulted from blocking CD28–CD80/CD86 interactions. Blockade of both CD2 and CD28 costimulation abrogated T cell proliferation and cytokine synthesis. Conversely, T cells stimulated with immobilized anti-CD3 and anti-CD2 mAb exhibited increased proliferation and IL-2 and IFN-γ expression, although a stronger enhancing effect was obtained with immobilized anti-CD3 and anti-CD28 mAb. Concurrent CD2 and CD28 costimulation caused a further increase in proliferation and cytokine synthesis. Stimulation of purified T cells with microsphere-immobilized anti-CD3 and anti-CD2 mAb increased IL-2 and IFN-γ mRNA stability. However, CD28 costimulation had a stronger enhancing effect on IL-2 and IFN-γ mRNA stability that was not further increased by concomitant CD2 signaling. CD2, therefore, costimulates T cell activation by stabilizing cytokine mRNA transcripts, albeit with less efficiency than CD28.