Meningiomas are common brain tumours which are generally benign, well circumscribed and slow growing. In a minority of patients complete surgical removal of the tumour is not possible, and re-growth of tumour tissue is a major clinical problem. The presence of receptors for progesterone in a large proportion of human meningioma tissues is well established. The occurrence of increased rates of growth of meningiomas during pregnancy suggests the existence of a relationship between high progesterone concentrations and the growth of meningiomas. These observations suggest that the use of antiprogestins may be of value in the treatment of meningiomas. However, experiments with cultured meningioma tissue (cells or explants) have shown only minimal effects of progesterone. It has been shown recently that many meningiomas have receptors for epidermal growth factor. We have investigated the response of cultured human meningioma cells to epidermal growth factor and the modulation of this response by progesterone and the progesterone-receptor blocking agent mifepristone (RU486). The results suggest that the presence of progesterone in the culture medium increases the sensitivity of meningioma cells to specific mitogenic stimuli without having direct mitogenic effects, whereas mifepristone can counteract the stimulating effects of progesterone.