Human pituitary prolactin (hPRL) contains a single major component. Upon iodination by a lactoperoxidase procedure approximately 50% of this component remains ulaltered in molecular size and net charge by the criteria of quantitative polyacrylamide gel electrophoresis (PAGE). Aminiotic fluid hPRL contains two components which remained ulaltered after iodination by the lactoperoxidase procedure used. Unaltered iodinated products can also be obtained from both sources by a stoichiometric chloramine T procedure, but in vastly diminished yield.