A major protein of human breast cyst fluid, termed GCDFP-24, has the property of specifically binding progestins. The purified glycoprotein, of 24,000 apparent molecular weight, bound pregnenolone and progesterone with highest affinity. The association constant for binding of progesterone was 1 X 10(6)L/mol by Scatchard analysis, and there was one binding site per molecule. Changes to the progesterone structure at C-17, C-20, or C-21 interfered with binding. The pH optimum for binding was 4-4.5. The purified protein was highly stable and was not irreversibly denatured by 50% methanol or 3M guanidine. However, dithiothreitol reversibly interfered with progesterone binding. Rabbit antiserum produced against the glycoprotein recognized an immunologically identical component in normal human sera, and partially cross-reacting components in normal monkey and baboon sera. The component in human sera was present in Cohn fractions IV and VI.