SUMMARYThe high plasma Cortisol and ACTH levels present in pregnant women as well as the non‐parallelism of their plasma extract dilution curves in comparison with the standard curve in the ACTH radioimmunoassay, are evidence for the presence of an ACTH‐like substance during pregnancy which would interfere with the assay.Placental extracts were obtained by acid‐acetone extraction, followed by partial purification with oxycellulose and by extraction with porous glass powder. A substance was detected which partially cross‐reacted with synthetic human ACTH in the radioimmunoassay and which showed biological activity using the assay procedure described by Lipscomb&Nelson. The data sustain the existence of an ACTH‐like placental hormone: human chorionic corticotrop