Apo D is localized to steroid-responsive tissues (adrenal cortex, ovary, endometrium, prostate and pituitary). Apocrine epithelium within axillary skin exhibited strongly positive staining for Apo D and peripheral nerves, pituitary and renal tubules were also immunoreactive. Apo D, a progesterone binding glycoprotein of 24,000 dalton monomer molecular size, is a constituent of high density lipoprotein in plasma and was first isolated in large quantity as GCDFP-24, the major protein component of most human breast cyst fluids.