ABSTRACT An albumin-like iodinated protein is described in the serum of two cases of anaplastic carcinoma of the thyroid gland, resembling the compound sedimenting with S20,w4 found in saline extracts of the malignant tissue of the patients. The 4S component of the extract contains human serum albumin, together with either an altered albumin or other 4S component. It is suggested that an intracellular defect is the cause of production of this component, and this is probably the source of the abnormal circulating iodinated compound.