This review analyzes recent data from international literature concerning the antiestrogen action of progesterone and progestins at the level of mammary cells in culture from either breast cancer lines or normal breast obtained from reduction mammoplasties. Most data indicate that progesterone and progestins have a strong antiestrogen effect on breast cell appreciated by the decrease of estradiol receptor content, the decrease of cell multiplication and the stimulation of 17β-hydroxysteroid activity which may be considered as a marker of breast cell differentiation dependent of progesterone receptor.