The response of CD8+ T cells to allogeneic tumor was studied by adoptive transfer of cells from TCR transgenic 2C mice specific for Ld alloantigen. Transferred cells were monitored during the course of a response to i.p. challenge with live P815 (H-2d) using the 1B2 mAb specific for the 2C TCR. Tumor was present in the draining LN and spleen within 3 to 4 days of challenge. The first changes in 1B2+ cells occurred in the spleen on day 4; VLA-4 expression increased in an Ag-specific manner and L-selectin expression decreased in an Ag-nonspecific manner. The number of 1B2+ cells in the spleen declined over days 4 to 6. The first detectable increase in CD25 expression and blast transformation was in the peritoneal cavity beginning days 5 and 6. Clonal expansion was largely limited to this site and was maximal on day 8. As expansion occurred in the peritoneal cavity; the number of 1B2+ cells in the draining LN and spleen also increased. These cells had an activated phenotype (CD44(high), VLA-4(high)) but most did not express CD25 and were not blasts. These results suggest that initial Ag recognition in the spleen results in altered expression of adhesion receptors so that cells gain access to the peritoneal cavity where they undergo clonal expansion and differentiation. Following the response, 1B2+ cells decline in number but a memory population (CD44(high), L-selectin(high and low)) persists for long times in the spleen and LN.