A messenger activity for HPL was identified in normal human term placentas. The mRNA was translated in rabbit reticulocyte cell-free system. The HPL synthesized was quantified by a specific immunoprecipitation and further identified by electrophoresis on sodium dodecyl sulfate polyacrylamide gel. The HPL synthesized in the reticulocyte lysate exhibited a molecular weight between 20,000 and 22,000 daltons similar to the active hormone. The messenger RNA activity for HPL corresponded to a sedimentation coefficient of 11-12 S. Furthermore the messenger activity for HPL was preferentially associated with membrane bound polyribosomes than with free polyribosomes.