The role of NK cells in the control of the metastatic spread of tumor cells was studied. Rats pretreated with rabbit anti-asialo GM1 (anti-asGM1) serum exhibited a diminished ability to destroy circulating MADB106 mammary adenocarcinoma cells, which in turn caused an increased incidence of experimental pulmonary metastasis. The anti-asGM1 treatment caused a selective inhibition of NK activity without detectable effect on T cell-mediated immunity, and overall had no effect on the cytotoxic activity or numbers of alveolar macrophages (alv.M phi) or monocytes. The suggestion of a role for NK cells in resistance to metastases from the MADB106 tumor cells was confirmed by the adoptive transfer of 5 X 10(6) highly purified large granular lymphocytes (LGL) into NK-depressed animals 2 hr before tumor challenge. This transfer of LGL, highly enriched in NK activity, partially or fully restored the ability of these rats to inhibit the development of pulmonary metastases. This ability to adoptively transfer resistance to metastases appeared to be confined to the LGL population, because transfer of the same number of mature peripheral blood T cells had no effect on tumor development. These results provide the first unequivocal evidence that LGL, with high NK activity, are involved in in vivo resistance to tumors, particularly in the elimination of potentially metastatic tumor cells from the circulation and capillary beds.