Bioassays have played an important role in determining the presence of hormones in the blood or other tissues as well as determining the biological potency of structural analogs of hormones. The frog skin bioassay, developed by Shizume et al. in 1954, has been particularly important in advancing our knowledge about the melanotropins. This bioassay, like the earlier assays for melanotropins, involved observations or recordings of the responses of pigment cells (chromatophores, particularly melanophores) present within the skin. Both in vitro as well as in vivo melanotropin bioassays have been employed in several vertebrate species. Melanoma cells provide a unique bioassay for determining the biochemical correlates of melanotropin action. In the present report some of the more commonly utilized bioassays for melanotropic peptides will be described. For a detailed discussion of the early bioassay systems for the melanotropins, several references are available, 2-6 in addition to the classical monograph by Parker. 7 For information on the radioimmunoassay of melanotropins several references are now available (see Wilson, Volume I, Chapter 11).