The chorionic somatommaotropic hormone extracted from the human placenta exists in several molecular forms. Analytical electrophoresis in polyacrylamide gel permits separation of a highly anodic migration form, form 1, and another form migrating slightly faster than albumin, form 2. These two forms are active as measured by radioactive immunological analysis, form 2 being about 25 times more active than form 1. The two forms are mutually interconvertible. The two forms may also be separated by filtration on Sephadex G-50. However, they do not differ in molecular weight, they have the same coefficient of sedimentation and the same coefficient of apparent diffusion, measured by analytic ultracentrifugation. Glutaraldehyde and 8 M urea do not modify their electrophoretic or chromatographic behavior. On the hand, the two forms differ in their tertiary structure, with the modification depending on the greater or lesser degree of oxidation of the intra-chain disulfide groups. The two forms also exist in placental culture media and the incorporation of tritiated leucine occurs preferably in form 1. The physiological significance of the two hormone pools is not clarified.