The amino acid sequences of the NH2-terminal extension prepeptides of the alpha and of the beta subunits of human choriogonadotropin have been determined by translation of first trimester placental mRNA in the presence of individually labeled amino acids. The structure of the prepeptide of the alpha subunit is: -24 -23 -22 -21 -20 -19 -18 -17 -16 -15 Met- Asp- Tyr- Tyr- Arg- Lys- Tyr- Ala- Ala- Ile -14 -13 -12 -11 -10 -9 -8 -7 -6 -5 Phe- Leu- Val- Thr- Leu- Ser- Val- Phe- Leu- His -4 -3 -2 -1 Val- Leu- His- Ser The structure of the prepeptide of the beta subunit is: -20 -19 -18 -17 -16 -15 -14 -13 -12 -11 Met- Glu- Met- Phe- Gln- Gly- Leu- Leu- Leu- Leu -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 Leu- Leu- Leu- Ser- Met- Gly- Gly- Thr- XXX- Ala In the presence of membranes, the beta prepeptide is cleaved to a protein containing the authentic NH2-terminal sequence of the secreted form of the beta subunit. Cleavage of pre-alpha subunit also yields a product containing the NH2-terminal region of the secreted form.