THE complete amino-acid sequences of pig and sheep adrenocorticotropic hormone have been elucidated1–3. They have almost identical structures showing only minor variations in the C-terminal region. A striking similarity in pattern occurs in the acidic and basic portions of these proteins, although these regions are comprised for the most part of different amino-acids. This pattern becomes obvious when the amino-acid sequence 24–35 of the acidic portion is listed directly beneath sequence 12–23 of the basic portion. The actual sequences for amino-acids 12–35 are shown in Table 1a and b for the two proteins. Table 1d is a hypothetical sequence compiled from the best pattern correspondence in both. The other portions of both corticotropins, sequences 1–11 and 35–39, are identical.