Monoclonal antibodies to human oestrogen and progesterone receptors were applied to frozen sections of normal endometrium in various phases of the menstrual cycle. Both oestrogen and progesterone receptors were localised within the nuclei of target cells. There are no oestrogen or progesterone receptors in endometrial vessels. Oestrogen receptor contents remained fairly constant during the proliferative and early secretory phases but decreased markedly in the late secretory phase. This decline was more obvious in the glandular cells than in the stromal cells, where receptor staining disappeared in most cases. The receptors were localised predominantly in the glandular cells at most stages of the menstrual cycle except in the late secretory phase, where the reverse was observed. In contrast, progesterone receptors rose gradually throughout the proliferative and early secretory phases followed by a decrease to various degrees depending on the cell types in the late secretory phase. This decrease was more marked in the glandular cells where again receptor staining disappeared in most cases, whereas the receptor contents in the stromal cells only decreased to the level of the late proliferative phase. The progesterone receptors were also localised predominantly in the glandular cells at most stages in the cycle apart from the late secretory phase.