A molydate-stabilized, 'non-activated' form of the progesterone receptor from the cytosol of oestrogenstimulated chick oviduct has been purified to homogeneity by a three-step procedure. The first step, affinity chromatography using a N-(l2-amino-dodecyl)-3-oxo-4-androsten-17~-carboxamide-substituted Sepharose gel, purified the receptor 1500 -2700-fold with z 50 % recovery. In the second step, ion-exchange chromatography through a DEAE-cellulose column, progesterone receptor was eluted as a single peak at 0.1 M KCI. Purification after this step was >6700-fold. The third step was filtration through Ultrogel AcA 34, resulting in overall purification z 7400-fold with overall recovery z 25 of pure receptor on the basis of 1 binding site/molecule of M, 85000. The purified molybdate-stabilized receptor had a sedimentation coefficient = 7.9 S & 0.1 ( n = 4) in 0.15 M or 0.4 M KCI containing sucrose 5-20% gradient and z 8.9 S k 0.2 (n = 6) in 0.15 M KCI containing glycerol 10-35 "/, gradient, and its Stokes radius was 7.05 k 0.10 nm (n = 3) (calculated M , between 240000 and 280000). Binding specificity of the purified receptor was the same as that found in crude cytosol. SDS-PAGE revealed a single band migrating as a polypeptide of MI z 85000 -+ 2300 (n = 9). PAGE under nondenaturing conditions at total acrylamide concentrations 5 "/,, 7 % and 9 "/, showed a single [3H]ORG 2058-protein band (ORG 2058 is a high-affinity analogue more suitable than progesterone for electrophoretic studies). The data suggest that the high molecular weight molybdate-stabilized progesterone receptor purified from oestrogen-primed chick oviduct is composed of only = 85000-M, polypeptide chains. [5] have recently described a 'non-activated' or 'native' form of the progesterone receptor from chick oviduct cytosol, stabilized and prepared in the presence of sodium molybdate. This form of the receptor sedimented at 8-9 S on sucrose gradients containing 0.15 M KCI, and was eluted from DEAE-cellulose as a single peak at 0.15 M KCI 151. Its Stokes radius R, was z 7.9 nm and data permitted calculation of M , z 290000. These studies prompted us to purify the molybdate-stabilized receptor using an appropriate biospecific affinity material. The chick oviduct progesterone receptor has been extensively studied by Schrader, O'Malley and coworkers during the past decade (for a review, see [6]), and two non-identical binding components have been characterized and separately purified from the cytosol of hen or of oestrogen-primed chick oviduct. These components were reported to be polypeptides of M, z 79000 ('subunit A') and "N I10000 ('subunit B'), respectively. They are readily separated by ion-exchange chromatography. In contrast, using crude cytosol, Murayama et al. [7] found MI = 84000 for a 'native' unit and Miller et al.[8] M , = 87000 for a 'fast' component of progesterone receptor from chick oviduct.We have developed a method to purify the molybdatestabilized form of the progesterone receptor. This involves a three-step procedure including a new aff...