The effect of RMI 10874 (RMI 10,874DA) in the abrogation of the metastatic spread of tumor cells is studied. Pre-treatment of BALB/c mice with the RMI 10,874DA completely eliminates lung colonization of an H-2-negative (GR9.B9) MCA-induced fibrosarcoma clone in an experimental metastasis assay^[1]. The effect of a tilorone analogue (RMI 10,874DA) and anti-asialo GM₁ serum is studied on the survival of BALB/c and C57B1/6 mice after i.v. injections of different syngeneic murine tumor cells. Natural killer (NK) cell activation in vivo by RMI 10874 prolongs survival and inhibits metastasis formation in mice, even when pretreatment consists of a single dose of the analogue^[2]. Tilorone is a small-molecule, orally bioavailable drug that is used clinically as an antiviral^[3].