The MET oncogene encodes a transmembrane tyrosine kinase receptor. Recently, hepatocyte growth factor (HGF), a potent growth factor for hepatocytes involved in liver regeneration, has been proposed as a ligand. In this paper, the physiological role of the human Met/HGF receptor is investigated by studying its specific distribution in normal and neoplastic tissues. Northern blot analysis has shown that the MET gene is selectively expressed in several epithelial tissues. High levels of MET mRNA have been found in liver, gastrointestinal tract, thyroid and kidney. Western blot analysis has shown that the levels of the Met protein generally correspond to those of the mRNA. However, in the thyroid, where there is a high level of MET mRNA, the protein was barely detectable, suggesting translational or post-translational regulation. The protein was also detected in the brain. Normal or increased levels of MET mRNA and Met protein were consistently found in fresh samples of carcinomas as well as in epithelial tumor cell lines. In thyroid carcinomas of a specific histiotype the amount of Met protein, almost undetectable in the normal counterpart, was found to be increased more than 100-fold. The tissue distribution of the Met/HGF receptor indicates that this molecule is involved in growth control of epithelial cells other than hepatocytes and suggests that its increased expression may confer a growth advantage to neoplastic cells.