Gamma-MSH (Melanocyte Stimulating Hormone), a putative hormone in the N-terminal region of the ACTH/beta- endorphin (beta-EP) precursor protein, was studied by RIA with an antiserum against gamma 3-MSH in ACTH-producing mouse pituitary tumor cells, AtT-20/D16v. Gamma-MSH peptides have low intrinsic melanotropic activity in mammalian melanoma cells, the specific pigmentary responses appear to be affected by the structure of the C-terminal portion.