Representative Sephadex G-75 elution profile of Day 15 uterine protein secretions. The protein was loaded onto a 72 X 2.6 cm column and eluted in 0.01 M PBS, pH 7.4 at 4°C. Fraction protein concentration was determined by the method of Lowry et al. (30) and analyzed by native-PAGE. Fractions 50-59 containing RBPs were pooled as indicated by brackets. Data shown here are from a single animal.