In addition to their use in pain control, opioids can function as regulators of tumor cell growth. We have found that the therapeutic opioid, methadone, significantly inhibits the in vitro and in vivo growth of human lung cancer cells, and this effect appears to be mediated by specific, high affinity, non-conventional opioid binding sites. The present study indicates the existence of multiple subtypes of binding sites mediating the peripheral and central nervous system actions of this drug. Pharmacological and biochemical characterizations of the methadone binding sites expressed in human brain and normal lung tissues indicate that these sites are distinct from each other and from other opioid receptor types present on human and rat brain membranes, as well as those expressed in human lung cancer cells. The identification of distinct methadone receptor types in the different tissues could lead to the development of more selective and less toxic drugs targeted toward the tumor cells.