The placental glycopeptide hormone chorionic gonadotropin (CG) is involved in establishing and maintaining pregnancy. The hormone consists of two different non-covalently associated subunits termed alpha and beta. In man there are seven closely linked genes coding for beta CG-like peptides, but only three of these appear capable of expression in the placenta. The organization of beta CG-like genes in man and baboon appears to be similar. We demonstrate here that the baboon genome contains multiple copies (at least five) of beta CG-related genes, and that these genes are closely linked in the genome. Nucleotide sequence data from several beta CG cDNA clones indicates that at least two of these beta CG-related genes are expressed in the baboon placenta. Analysis of beta CG sequences from baboons and human subjects demonstrates that these genes have diverged markedly between species. In contrast, these sequences are remarkably homogeneous within their respective genomes. Gene conversion events may be responsible for retaining the high degree of identity among the various beta CG gene family members. Knowledge of beta CG sequences from baboon may lead to development of a long-term antipregnancy vaccine. The ability of CG antibodies to interfere with the maintenance of pregnancy can now be investigated within a homologous system.