We have studied memory in T cell receptor (TCR) transgenic mice expressing a Db-restricted TCR specific for the male peptide (H-Y). CD8+ T cells from female TCR transgenic C57BL/6 (B6) mice were activated by transferring them into X-irradiated male (B6 x bm12)F1 hybrid recipients. Subsequently, they were highly purified by cell sorting and transferred for various lengths of time into female B6 nu/nu recipient mice. Other nu/nu recipient mice received highly purified naive T cells expressing the transgenic TCR. The functional potential of naive and "memory" T cells was analyzed by stimulation with male cells in vivo. The results show that memory cells can be derived from activated T cells and persist in the absence of antigen for at least 13 weeks. Naive and memory T cells differ in that memory T cells give a more vigorous and sustained response than naive T cells.