The authors have sought the existence of cellular hyper-receptivity in susceptible cells in order to explain the transformation of normal uterine muscle fibres into tumours, because of the absence of true biological proof of hyperoestrogenization in fibromyomata. The total of uterine receptors for oestradiol (the total number of accessible receptors or those in which endogenous oestrogens had been taken up) were measured in normal myometrium and in fibroids removed at myomectomy or at hysterectomy. The oestradiol receptors are significantly raised in fibromyomatous tissue when the results are expressed as femtomoles per mg. of protein. All the same, there is no significant difference to be found when the results are expressed in micrograms of DNA, which leads to the supposition that there is no significant difference in the concentration of oestrogen receptors in the two types of cells. The cause of the cellular multiplication of fibromyomatous tissue should therefore be sought at another level.