The effect of murine α/β interferon (IFN) on experimental metastasis was investigated using B16-F10 melanoma cells. Since the outcome of metastasis of blood-borne tumor cells is mainly determined within the first 24 h after i.v. inoculation of tumor cells, i.p. injection of IFN was focused on this critical early phase. The inhibition of pulmonary metastases by IFN was found to be maximal when given 3 h prior to tumor cell inoculation, while mice with 24-h and 12-h pretreatment and simultaneous IFN treatment also showed a reduction in metastases, but to a lesser extent. However, mice receiving IFN 2 h after tumor cell inoculation did not show any reduction. Tumor cells cultured for 24 h in IFN-containing medium showed no reduction in metastases. Administration of anti-asialo GMl prior to IFN treatment was found to eliminate the inhibitory effect of IFN 3 h pretreatment. However, natural killer (NK) cell activity in vitro measured at 3 h, 13 h and 24 h after IFN administration was enhanced to the same extent, not paralleling the inhibitory effect on pulmonary metastases. These data indicate that prepared host status against blood-borne tumor cells is established by IFN pretreatment, being maximal when injected several hours prior to tumor cell inoculation, and that this effect is substantially dependent on NK cell activity, though the implication of other factors is not excluded.