In this study, an electrochemical immunosensor for milk progesterone produced by dairy cows was developed. Using the immunosensor, milk progesterone levels in healthy estrus dairy cows was found to range from 1 to 6 ng/mL 20 days after estrus. there were high levels of progesterone in the milk from cows with prolonged luteal phase and luteal cysts, which ranged from 15 to 28 and 19 to 29 ng/mL, respectively. Cows with inactive ovaries also showed low milk progesterone levels of 1-8 ng/mL, but they had lower plasma follicle-stimulating hormone (FSh) levels than the healthy estrous cycle dairy cows.