Previously we have shown that intravenous injection of male B6 lymphocytes containing CD8+ cells into B6 female anti-HY TCR transgenic mice results at 6 wk in the disappearance of the majority of male Ag-reactive T cells (TghighCD8+) from the periphery. Here we investigate the process in more detail. B6 female anti-HY TCR transgenic mice were intravenously injected with viable lymphocytes from male B6 normal, CD4-/-, CD8-/- or CD8-/- carrying a CD8 transgene lacking its cytoplasmic tail ("CD8 tail-less") mice. The fate of TghighCD8+ cells was followed in vivo. There was always a large (at least twofold) expansion of these cells in the periphery 4 days after encountering male Ag. Their subsequent fate differed, however, depending on whether or not the injected male lymphocytes contained normal CD8-expressing cells. If the injected male lymphocytes contained normal CD8 cells, at 6 wk there was a large drop in the number of TghighCD8+ cells associated with a rise in the number of TghighCD8- cells. If the injected male lymphocytes lacked CD8 cells or expressed only tail-less CD8, TghighCD8+ cell numbers returned to normal by 6 wk, while TghighCD8- cell numbers remained unchanged. The TghighCD8- cells, although carrying high levels of the male specific TCR, did not react to male Ag. In addition, their ability to respond in anti-CD3-induced activation, which does not require CD8 as a co-receptor, was significantly impaired. Our data suggest that down-regulation of CD8 on the Ag-reactive T cells accounts, at least partially, for the disappearance of HY-reactive T cells from the periphery. Further, some step in the process requires signaling through the cytoplasmic domain of CD8 on the injected Ag-bearing cells.