Tumor-specific effector T cells (T E) are naturally sensitized within the L-selectin low (CD62L low) fraction of tumor-draining lymph nodes (TDLN). Whether isolated from day 9 (D9) or day 12 (D12) TDLN, 5 million L-selectin low T E could be culture activated and adoptively transferred to achieve complete rejection of established intradermal, pulmonary, and brain tumors. Surprisingly, although 25 million unfractionated T cells from D9 TDLN were equally effective, even 100 million unfractionated T cells from D12 TDLN seldom prevented lethal intradermal tumor progression, despite a pronounced therapeutic excess of T E. This highly reproducible treatment failure was due to cotransfer of tumor-induced, L-selectin high suppressor T cells (T S) which were also present in D12 TDLN. In contrast, D9 TDLN and normal spleens lacked L-selectin high T S. Only those L-selectin high D12 TDLN T cells that down-regulated L-selectin during culture activation were suppressive in vivo and in vitro, and, like L-selectin low T E, trafficked promptly into tumors following iv administration. This is the first demonstration that adoptive immunotherapy can fail as a direct result of passenger T S that share certain phenotypic and trafficking features of T E, even when otherwise curative doses of T E have been administered. Furthermore, in contrast to recently described CD4+ CD25+ T S and plasmacytoid dendritic cell-activated T S, tumor-induced L-selectin high T S prevent tumor rejection via blockade of sensitized, activated T E rather than via afferent blockade.