Over-expression of the oncogenic receptor tyrosine kinase (RTK) Xmrk is sufficient to induce formation of hereditary malignant melanoma in the fish Xiphophorus. In the melanoma tissue as well as in a melanoma-derived cell line (PSM), the Xmrk protein shows strong tyrosine phosphorylation, indicating either ligand-independent or autocrine activation of its kinase domain. However, it is unknown whether the constitutively activated Xmrk receptor itself directly triggers the proliferative signals, thus leading to uncontrolled growth of the pigment cells. In order to evaluate the role of Xmrk in proliferation of melanoma cells, we inhibited its kinase activity by using a Xmrk specific tyrphostin. At a concentration of 10 microM, tyrphostin AG555 led to a decrease of the Xmrk-induced DNA synthesis to 10% in NIH 3T3 Hm cells, whereas serum dependent 3H-thymidine incorporation was unaffected. In fish melanoma cells, the drug efficiently blocked DNA synthesis and cellular growth. This anti-proliferative activity correlated with the potency of AG555 to inhibit Xmrk autophosphorylation, indicating that the Xmrk receptor is the major determinant of mitogenic signaling in Xiphophorus melanoma cells.