OGF is the specific opioid peptide involved in the growth inhibition of human anaplastic thyroid cancer cells. (A) The effects of various endogenous and exogenous opioids (10-6 M) or sterile water (Co) on KAT-18 cell number. (B) Cells were treated with an antibody specific for OGF (CO172), pre-immune serum (IgG), exogenous OGF (10-6 M), or sterile water (Co). Cell number was measured at 72 h. Data for both experiments represent mean SE for at least 2 aliquots/well from 2 wells/group. Three experiments were conducted for each measure. Significantly different from respective controls at p < 0.01 (**) or p < 0.001 (***).