The mu₃ opiate receptor subtype has been characterized by various binding assays as opiate alkaloid selective (e.g. morphine) and opioid peptide (e.g. methionine enkephalin) insensitive. This opiate receptor subtype has been found on human, including cancer cell lines, and invertebrate tissues, demonstrating that it has been conserved during evolution. Furthermore, in numerous reports, this receptor is coupled to constitutive nitric oxide release. In this regard, for example, morphine immune down regulating activities parallels those actions formerly attributed to nitric oxide. We have now identified the mu₃ receptor at the molecular level and sequence analysis of the isolated cDNA suggests that it is a novel, alternatively spliced variant of the mu opiate receptor gene (MOR). Furthermore, using Northern blot, reverse transcription coupled to polymerase chain reaction (RT-PCR) and sequence analysis, we have demonstrated the expression of this new mu variant in human vascular tissue, mononuclear cells, polymorphonuclear cells, and human neuroblastoma cells. The presence of this mu splice variant, adds to the growing body of evidence supporting the hypothesis that morphine is an endogenous signaling molecule in neural, immune and vascular systems. In addition to their use in the treatment of pain, opioid peptides appear to be important in the growth regulation of normal and neoplastic tissue. This review will focus on the influence of opiate alkaloids, e.g., morphine, on tumor growth, with emphasis on immuno-regulatory and antiproliferative mechanisms.