Chorionic gonadotropin (CG), a pregnancy-specific heterodimeric hormone found in primates, is responsible for CL rescue with pregnancy maintenance. Of the primates, the human and baboon gene sequences are the only structures so far determined. In order to study the structure and function of CG in other primates, we have isolated and sequenced the coding regions for the two subunits of marmoset CG (mCG) by the reverse transcription/polymerase chain reaction method. Study of multiple clones confirmed a high degree of homology with the human sequences (88% and 80% for the alpha and beta nucleotide sequences, respectively). Marmoset CGα has an extra four amino acids compared to hCGα, whereas the mCGβ sequence has a 3-bp deletion that maintains the reading frame and C-terminal amino acid sequence. Most of the differences between hCGβ and mCGβ peptides occur in the C-terminal region, which includes the loss of two of the O-linked glycosylation consensus sequences and the presence of an N-linked glycosylation consensus sequence. When mCGα and β were co-expressed in CHO cells, assembly of biologically active hormone was confirmed by induced steroid secretion by MA10 cells. Partially purified mCGβ was used to raise anti-mCG antibodies. To date, an antibody has been obtained that is capable of detecting recombinant mCGβ, recombinant mCG dimer, and mCG dimer secreted by cultured marmoset trophoblast. Marmoset CGα and β were also detectable at the transcriptional level in cultured trophoblast by in situ hybridization. This suggests that the LH/CG bioactivity reported from marmoset placentae and embryos is due to a molecule with structural features common to hLH (glycosylation pattern) and hCG (CGβ C-terminal structure).