The residues 90-92 can be split off from the C-terminal region of the isolated alpha-subunit of choriogonadotropin (residues 88--92: -Tyr-Tyr-His-Lys-Ser-OH) by means of serine carboxypeptidase (des-Lys91,Ser92-alpha-subunit; des-(90-92)-alpha-subunit). However, when choriogonadotropin is digested by serine carboxypeptidase, only the residues 143-145 (-Leu-Pro-Gln-OH) form the C-terminus of the beta-subunit are released (des-(143-145)-choriogonadotropin). Depending on the pH conditions, glutamine 145 and the residues 143-145, respectively, are liberated by digestion of the isolated beta-subunit (des-Gln145-beta-subunit and des-(143-145)-beta-subunit, respectively). The present study provides evidence that the C-termini of both the isolated subunits and those in choriogonadotropin are probably arranged on the surface of the molecules. The biological activity of des-(143-145)-choriogonadotropin is not significantly decreased. The immunological activity, however, is reduced when measured by complement fixation. In comparison to the native hormone, a four-fold amount of des-(143-145)-choriogonadotropin has to be applied to obtain highest complement fixation. The conformation of des-(143-145)-choriogonadotropin does not seem to differ from that of the native hormone, when estimated both by CD measurements and by Ans-choriogonadotropin fluorescence. The respective determinant therefore seems to depend, at least to some extent, on the sequence of the C-terminal region of the beta-subunit of the hormone; complement fixation, however, does not seem to be affected significantly, when the des-(143-145)-beta-subunit is compared with the native beta-subunit using an antiserum against the native beta-subunit. This provides evidence that this C-terminal determinant is possibly more immunogenic at the hormone than at the isolated beta-subunit. The biological activity of recombined choriogonadotropin in vivo as well as in vitro is markedly reduced when serine 92 is removed from the C-terminus of the alpha-subunit (des-Ser92,Lys91-alpha-native beta-subunit: 36% residual activity in vivo). Biological activity is lost when the residues 88-90 are removed by digestion of the des-Ser92,Lys91-alpha-subunit with carboxypeptidase A. Recombination products between a modified alpha-and the native beta-subunit show a reduced Anschoriogonadotropin fluorescence (des-Lys91,-Ser92-alpha + native beta-subunit: 52%; des-(88-92)-alpha- + native beta-subunit: 23%). The Ans-induced aggregation of choriogonadotropin, however, also takes place in those recombination products which display a low Ans-choriogonadotropin fluorescence, indicating that the reduction is probably not caused by a portion of the molecules losing their binding sites for Ans. Therefore the diminished Ans-choriogonadotropin fluorescence seems to signal small conformational changes. The CD spectra of the native and the des-(90-92)-alpha-subunit, however, seem not to differ significantly. It is shown that the release of amino acids from the C-terminus of the alpha-subunit causes a disturbance of the interaction between the subunits. This seems to prevent an effective conformational change of the beta-subunit which probably is a prerequisite for the binding of the hormone to the receptors of Leydig cells.