The most frequently studied tumour in a target organ for steroid activity has been breast cancer in women. The hormone-dependent nature of these tumours is now accepted as being related to the presence of oestradiol and progesterone. The fact that these receptors are found in mammary tumours might be connected with one aspect of cell differentiation which bears a close relationship to slower clinical progression. Hormonal manipulations are now employed routinely to treat other tumours of these target organs: endometrium, prostate, kidneys. Receptors of this type have been demonstrated in other solid tumours (melanoma) but hormonal treatment has given disappointing results. Tumours in organs that are not typical targets for these hormones do contain steroid receptors, but their significance in relation to cancer induction and growth phenomena has not been elucidated.