Endogenous opioid peptides serve as growth factors in normal and neoplastic cells and tissues, and both opioids and their receptors have been identified in human colon cancer. This study examined the hypothesis that opioids serve to modulate the growth of human colon cancer. Daily administration of the native opioid growth factor (OGF), [Met5]enkephalin, at dosages of 0.5, 5, or 25 mg/kg prevented the occurrence of human colon cancer HT-29 xenografts in nude mice. More than 80% of the mice receiving OGF beginning at the time of tumor cell inoculation did not exhibit neoplasias within 3 wk, in comparison with a tumor incidence of 93% in control subjects. Even 7 wk after cancer cell inoculation, 57% of the mice given OGF did not display a tumor. OGF delayed tumor appearance and growth in animals developing colon cancer with respect to the control group. The suppressive effects of OGF on oncogenicity were opioid receptor mediated. OGF and its receptor, zeta (zeta), were detected in transplanted human HT-29 colon tumors. Surgical specimens of human colon cancers also contained OGF. These results show that a naturally occurring opioid peptide acts as a potent negative regulator of human gastrointestinal cancer and may suggest pathways for tumor etiology, progression, treatment, and prophylaxis.