The antimetastatic effects of heparin (40 units) and prostacyclin (PGI2, 100 microgram)1 were investigated in normal mice and in mice with depressed or activated natural killer (NK) cell activity. Both anticoagulants inhibited the formation of lung metastases after inoculation of the FI or F10 sublines of B16 melanoma. Inhibition of NK activity by treatment of mice with anti-asialo GM1 serum abrogated the antimetastatic effects of PGI2 or heparin. Conversely, augmentation of NK-cell activity by poly I:C plus treatment with anticoagulants produced synergistic antimetastatic effects. A similar pattern of results was obtained with heparin treatment of mice challenged with the Madison lung carcinoma (M109), but PGI2 alone or in combination with theophylline had little or no detectable antimetastatic effect on M109 or on the parental B16 melanoma. Studies of the mechanism of the interaction between heparin nd NK cells revealed that the anticoagulant treatment did not affect splenic NK activity in vitro. However, heparin treatment caused a significant increase in the clearance of radiolabelled tumor cells from the lungs of normal mice. Combined treatment of mice with poly I:C and heparin synergistically accelerated the elimination of radiolabelled tumor cells. In contrast, heparin did not affect the clearance of tumor cells from the lungs of mice with depressed NK activity. Thus the antimetastatic effects of heparin and PGI2 are dependent on levels of NK activity in the host. Platelet aggregation and fibrin coating of the surface of tumor cells may be among the mechanisms by which hematogenously spread tumor cells are protected from destruction by NK cells. Anticoagulant drugs may exert antimetastatic effects by making tumor cells more vulnerable to the cytotoxic effects of NK cells, rather than by blocking adherence of tumor cells to vascular endothelium.