The present manuscript demonstrates that B16F10 melanoma cells activate the enzyme acid sphingomyelinase in thrombocytes via the surface molecule P-selectin, by which ceramide is released. Metastasis of tumor cells in the lung is decreased by up to 95% by genetic deficiency of P-selectin molecule or deficiency of acid sphingomyelinase. After activation of wild type thrombocytes by B16F10 melanoma cells there is a rapid increase in acid sphingomyelinase activity and ceramide production as compared to acid sphingomyelinase-deficient thrombocytes or P-selectin-deficient thrombocytes. A lack of interaction of B16F10 melanoma cells and thrombocytes was excluded by activation of PLCγ, JNK and MAP kinase, indicating that these signaling events are stimulated in both, wild-type and P-selectin-deficient platelets, proving that B16F10 melanoma cells interact with and activate P-selectin-deficient thrombocytes. The molecular mechanisms of tumor metastasis are currently fairly incomplete, though metastasis plays a crucial clinical role in cancer patients. Acid sphingomyelinase is iidentified as a novel target molecule for the inhibition of tumor metastasis. In order to pharmacologically inhibit the thrombocytic P-selectin system, an intravenous injection of fucoidan showed a decrease of tumor metastasis of B16F10 melanoma cells by approximately 75%. This indicates that tumor metastasis can be blocked pharmacologically, which is of great clinical interest.