To determine whether ectopic secretion of a protein hormone can occur normally, we studied plasma from normal, nonpregnant subjects for the presence of a placental hormone, human chorionic gonadotropin. We extracted and purified this hormone from other plasma proteins. We identified the hormone in the final residue on the basis of its dose-response curves in a specific radioimmunoassay and calculated the plasma concentration after correction for losses. Because this assay is sensitive to concentrations as low as 2 pg per milliliter, human chorionic gonadotropin could be detected in the plasma of 12 of 16 blood donors; the median concentration was 19 pg per milliliter (range, less than 2 to 361). This immunologic human chorionic gonadotropin was further characterized from a pool of normal plasma by gel filtration on Sephadex G-100 and was found to be identical to the standard form of the hormone. The concentration in this pool from 13 normal men was 18 pg per milliliter. The source of this ectopic hormone production is unknown, but may be normal, rapidly proliferating nonmalignant cells.