Nude mice inoculated with human pancreatic cancer (BxPC-3) cells and receiving 5 mg/kg of opioid growth factor ([Met5]enkephalin; OGF) three times daily exhibited a marked retardation in tumorigenicity compared to animals injected with sterile water (controls). OGF-treated animals had a delay of 43% in initial tumor appearance compared to control subjects (10.6 days). At the time when all of the control mice had tumors, 62% of the mice in the OGF group had no signs of neoplasia. Tumor tissue excised from mice after 30 days was assayed for levels of [Met5]enkephalin and zeta opioid receptors. Tumor tissue levels of [Met5]enkephalin were 24-fold greater in OGF-treated mice than controls, but plasma levels of OGF were 8.6-fold lower in animals receiving OGF. Specific and saturable binding of radiolabeled [Met5]enkephalin to nuclear homogenates of pancreatic tumor tissue was recorded, with a binding affinity (Kd) of 10 nM and a binding capacity (Bmax) of 46.8 fmol/mg protein. Binding capacity, but not affinity, of [3H-Met5]enkephalin was reduced by 58% of control levels in tumor tissue from mice of the OGF group. OGF and the zeta (zeta) opioid receptor were detected in human pancreatic tumor cells by immuno-cytochemistry. These results demonstrate that an endogenous opioid and its receptor are present in human pancreatic cancer, and act as a negative regulator of tumorigenesis in vivo.