There is growing evidence to indicate that pituitary and placental glycoprotein hormones are comprised of non-identical subunits. Several of these hormones have now been resolved into their constituent subunits. The hormones derived from several species appear to share a polypeptide subunit (α), with identical, or nearly identical, characteristics. Present studies suggest that the biological properties of the native hormones may be predominantly contributed by the β subunit, which also carries specific antigenic determinants, and is structurally unique for each species.