AbstractAmmonium sulfate (AS) precipitation by consecutive steps at 10% to 50% saturation has been utilized for fractionation of cytoplasmic estrogen receptor (ER) and progesterone receptor (PgR). The highest percentage of both receptor activities are confined mainly in two fractions at AS saturation from 20% to 30% and 30% to 40%. The total percentages of activity of the cytoplasmic ER and PgR salted out at 50% saturation are 77% and 53%, respectively. Precipitation of hormone-receptor complexes at 50% saturation for assay of ER and PgR can be achieved, but needs improvement for efficient salting out of the receptors. ER and PgR salted out in the AS pellet are much more stable for storage than in the cytosol.