We have investigated the response of T cells to staphylococcal enterotoxin A (SEA) injections in vivo. We found that a single injection of SEA with an optimal dose of 10 μg increased the expression of both CD4 and CD8 significantly. There was expansion of SEA-reactive T cells in vivo after SEA re-injection and the time interval between injections strongly influenced the responsiveness of CD4+ and CD8+ T cells.Anergy of T cells was observed after three SEA treatments. The time interval between injections mainly affected the unresponsiveness of CD4+ T cells, not CD8+ T cells. Marked deletion followed by anergy of CD4+ T cells was induced at short intervals, and anergy without obvious deletion of CD4+ T cells was induced at long intervals. We also found that the anergic state was reversible in vivo. Repeated SEA stimulation led to down-regulation of interleukin (IL)-2, and high levels of IL-10. This study showed that both CD4+ and CD8+ SEA-primed T cells were responsive to SEA rechallenge in vivo, and a third injection was needed to induce the anergy of T cells.