Progesterone from the ovarian corpus luteum plays a key physiological role in animal reproduction. Knowledge of progesterone levels can provide valuable information for reproductive management of dairy cattle. Milk progesterone monitoring offers an accurate and objective measurement of factors associated with postpartum ovarian activity. The dose response curve of the Progesterone-EIA shows a measuring range of 0.1 – 5 ng/ml. Defatted milk samples can be analyzed directly for the concentration of progesterone at the relevant levels between 0.1 – 5 ng/ml.