We have used restriction enzyme analysis to sample similarities and differences among eight cloned genes, isolated from two independent libraries, which hybridize to a cDNA for the beta subunit of human chorionic gonadotropin (hCG). We find that one of the cloned genes encodes the beta-subunit of the closely related hormone, human luteinizing hormone (hLH). Using differential hybridization of human DNA with beta hCG- and beta hLH-specific probes, we show that there are seven beta hCG genes but only a single beta hLH gene in the human genome. This single beta hLH gene is linked to at least three beta hCG genes. Our restriction analysis reveals that the seven beta hCG genes are extremely similar but not identical, and that there are no obvious structural rearrangements to indicate which, if any, are pseudogenes. These data also reveal that the beta hLH gene is far more homologous to the seven beta hCG genes than the beta hCG:beta hLH protein sequence homology of 82% might predict. We propose that the beta hCG gene arose by a duplication of an ancestral beta hLH gene and that, once the beta hCG function was established, the ancestral beta hCG gene itself duplicated and rearranged, creating the present organization of multiple beta hCG genes.