Normal syngeneic spleen cells were found to inhibit the local growth of the Lewis lung carcinoma (3LL) when injected together with the tumor cells at a ratio of 100:1 (spleen to tumor cells). The repeated injection of the tumor cells together with spleen cells eventually led to the selection of a tumor cell population whose growth could no longer be inhibited by normal spleen cells. In a previous report from this laboratory, a tumor subpopulation obtained in this manner was shown to display an increased metastatic potential, as well as a decreased sensitivity to natural resistance mechanisms in vivo and NK lysis in vitro. In the present study, we attempted to characterize the spleen cell population which mediated this selection process. We found that spleen cells depleted of T cells, B cells, or adherent macrophages retained their ability to inhibit tumor growth and select a resistant line in vivo. Subsequently, two tumor sublines derived by continuous in vivo passage of the parental tumor line with either unfractionated or nylon wool-non-adherent spleen cells were characterized. It was found that whereas both sublines were resistant to growth inhibition by normal spleen cells, only the subline derived from continuous passage with unfractionated spleen cells showed a reduction in the density of H-2b molecules expressed on the cell surface and an enhanced metastatic potency. These results suggest that the resistance of a tumor line to natural killer cells may not always result in an increase in its metastatic potential.