Conditions are described for the preparative isoelectric focusing in flat beds of Sephadex of the progesterone receptor from the chick oviduct. The method allows the fractionation of the receptor into two molecular species, one focusing at pI 6 and the other at pI7 with good purification and recovery. The pI 6 and pI 7 receptor species were purified 2- and 26-fold, respectively. The assaying of the focused fractions with the charcoal binding method provides an accurate identification and quantitation of the [3H] progesterone receptor. The method is reproducible in recovery, quantitation, and resolution of the two receptor species. The receptor with an apparent pI of 6 sediments at S on linear sucrose gradients, while the receptor with an apparent pI of 7 sediments at ~3.5 S. On the basis of the sedimentation values and elution patterns from diethylaminoethyl (DEAE) chromatography, the pI 6 component is equivalent to the “B” receptor species and the pI 7 component is equivalent to the “A” receptor species described previously [Schrader, W. T., & O'Malley, B. W. (1972) J. Biol. Chem. 241, 51-59].
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