The immune response to a murine myeloid leukemia (cell line C1498) was studied in vitro and in vivo. Natural killer (NK) cells and CD8+ cytotoxic T lymphocytes (CTL) were shown to lyse C1498 in vitro through the binding of leukocyte function antigen-1 (LFA-1) on effectors and intercellular adhesion molecule-1 (ICAM-1) and ICAM-2 on C1498 target cells. However, the ability of nonimmunized mice to resist an in vivo challenge of a low dose (10(4)) of C1498 was NK-cell, but not T-cell dependent. The failure of T cells to participate in the immune surveillance of a low leukemia burden appeared, in part, because of a lack of expansion of leukemia reactive CTL precursors (CTLp). Leukemia reactive CTLp frequency estimations in naive and leukemia bearing mice were not significantly different (range, 1:20,600 to 1:74,000) in contrast to immunized mice (range, 1:1,400 to 1:4,400). Leukemia reactive CTLp could be expanded to a level that could apparently mediate in vivo immune surveillance of 10(5) leukemia cells by injection of irradiated leukemia cells intraperitoneally (IP) or subcutaneously (SC), but not intravenously (IV). However, IV injection of 10(5) live leukemia cells engineered to secrete interleukin-2 (IL-2) resulted in systemic immunity mediated primarily by CD8+ T cells. We conclude that NK cells can mediate immune surveillance of a low leukemia burden. CD8+ CTL-mediated immune surveillance can eliminate a higher leukemia burden than NK cells, but requires T-cell help, which can be delivered by local IL-2. Both NK and CTL-mediated immune surveillance of C1498 murine myeloid leukemia is dependent on recognition through the LFA-1:ICAM adhesion pathway.