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-This study examines cell turnover within the lobules of the "resting" human breast and correlates it to the stage of the menstrual cycle. The results are based on the morphological identification of both cell multiplication (mitosis) and cell deletion (apoptosis). It is found that these events undergo significant cyclical changes during the menstrual cycle, with raised levels towards the end of the cycle and during menses. However, in relation to a 28-day menstrual cycle, the position of the mitotic and apoptotic peaks, at Days 25 and 28 respectively, are significantly different. The high values are associated with an increase in the number of lobules showing a slight response rather than a large reaction within a few lobules. It appears that the "resting" breast tissue shows a general, rather than a focal reaction to a given hormonal environment. The possible role of oestrogen and progesterone as effectors of these changes is discussed. Our results show that the menstrual cycle influences cell turnover, though different factors may be affecting mitosis and apoptosis.