A close correlation was demonstrated between levels of host natural cytotoxic (NC) and/or natural killer (NK) cell activity and capacity to eliminate blood-borne tumor cells. The outcome of experimental metastasis of several tumors with defined biologic behavior was studied in syngeneic mice exhibiting low NK cell activity [3-wk-old normal mice and cyclophosphamide (Cy)-treated adult mice] and high NK cell activity (normal adult mice). An iv injection of metastatic tumor cells into 3-week-old or Cy-treated mice markedly enhanced experimental pulmonary metastasis. The increased incidence of metastasis in mice exhibiting low activity of NK cells was not due to enhanced tumor cell arrest in the lung but rather to increased tumor cell survival. Boosting the NK activity of 3-week-old, but not Cy-treated, mice with interferon inducers inhibited metastasis formation. The adoptive transfer of spleen cells from syngeneic mice or allogeneic nude mice that have high NK activity shortly before (but not after) iv tumor challenge abrogated the Cy-induced enhancement of metastasis. The reactive lymphoid cells were non-T, nonadherent to nylon wool, sensitive to Cy treatment, and endowed with a natural ability to kill tumor cells during a short (12–24 hr) period. The conclusions were that NC-NK cells are important in host defense against circulating tumor cells and therefore can prevent the development of tumor cells into metastases.