[en] We have added nine extra residues to the C-terminal of human prolactin and analysed the effect of this mutation on the ability of the hormone to bind to its lactogenic receptor and to induce Nb2 cell division. Both properties are markedly affected when compared to the natural 23-kDa human prolactin. Since no alteration of the global protein folding was detected either by circular dichroism or by infrared spectroscopy, the decrease in biological potency can be exclusively attributed to an effect of the nine additional residues on their near environment. From infrared analysis and secondary structure prediction, the elongated tail is assumed to be involved in a beta-sheet with a few residues initially belonging to the fourth helix. Moreover, from the X-ray structures of porcine and human growth hormones, two proteins homologous to prolactins, the nine extra residues are likely to fold within a concave pocket delimited by helices 1 and 4, and the second half of the loop connecting helices 1 and 2 (loop 1). Thereby, we suggest that the additional residues prevent some residues belonging to this pocket from interacting with the lactogenic receptor. This is in perfect agreement with our earlier proposal that the binding site of prolactin to the lactogenic receptor is homologous to that of growth hormone to the somatogenic receptor, i.e. essentially composed of residues belonging to this concave pocket.