Hepatocyte Growth Factor (HGF) receptor, encoded by the protooncogene c-met, is overexpressed in many human tumours, including those of thyroid epithelium. The absence in most cases of any primary structural abnormality of the met gene suggests that overexpression is secondary to mutation of other gene(s). To test this hypothesis we investigated the effect on met expression of two activated oncogenes known to play a major role in thyroid oncogenesis, ras and ret. To minimize the possibility of unknown co-operating events, we introduced these genes directly into normal human thyrocytes in primary culture using amphotropic retroviral vectors and assessed met expression as early as possible in the resulting epithelial colonies. Double immunofluorescence revealed expression of met protein, strictly localized to cells expressing the mutant ras and ret vectors, expression in background normal cells being barely detectable. In contrast, colonies induced to proliferate at a comparable rate by a vector expressing SV40 T showed no increase in met expression. To permit quantitation by Western blotting we also extended these findings to a thyroid cell line (R18) containing a zinc-inducible mutant ras gene. Induction of the oncogene led to a fourfold increase in met protein expression. We conclude that overexpression of met is induced by activation of the ras or ret signalling pathway and not simply by deregulation of the cell cycle per se. The data suggest that the proliferative advantage conferred by these oncogenes may be in part due to the resulting sensitization of tumour epithelium to paracrine HGF secreted by stromal cells.