Squamous cell carcinoma of the head and neck (SCCHN) is the sixth most common malignancy worldwide. Approximately half of the patients afflicted die within 5 years of diagnosis, and surviving patients may be left with severe esthetic and functional compromise. In this study, we discovered that an endogenous opioid peptide, [Met5]-enkephalin, inhibited the growth of human SCCHN in vitro; in view of this pentapeptide's action it has been termed opioid growth factor (OGF). OGF was found to be a constitutively expressed, receptor-mediated growth inhibitory agent that appears to be autocrine produced and secreted. Growth regulation was dose-related, reversible, cytostatic, and independent of serum. All 6 human SCCHN cell lines examined exhibited growth modulation by OGF. Blockade of peptide-receptor interaction by opioid antagonists (naltrexone), or addition of antibody to OGF, resulted in substantial increases in cell number compared to control levels, showing the tonic nature of OGF-zeta activity. Immunocytochemical studies detected both OGF and its related receptor, zeta, in these cells, correlating with earlier findings of peptide and receptor in specimens of SCCHN obtained at surgery. These data suggest that a native opioid peptide, OGF, interacts with a novel opioid receptor, zeta, to tonically arrest the growth of human SCCHN.