Abstract: The present invention relates to pharmaceutical compositions for treating neoplasias in an animal or human comprised of a carrier and therapeutically effective amounts of at least one chemotherapeutic agent along with the biotherapeutic endogenous pentapeptide Met-enkephalin, referred to as opioid growth factor. Also provided are methods of treating neoplasias in an animal or human in need of such treatment, comprising the administration to the animal or human therapeutically effective amounts of a pharmaceutical composition comprised of a carrier and therapeutically effective amounts of at least one neoplasia-treating agent, such as a chemotherapeutic agent or radiation, along with opioid growth factor.