Steroid--fluorescein amine and steroid-BSA-fluorescein-isothiocyanate conjugates have been prepared and their ability to bind to oestrogen receptors assessed in competitive binding studies. The binding of all the fluorescent conjugates to uterine cytosol proteins was low when compared with either oestradiol or diethylstilboestrol. A comparative study was carried out to assess the relationship between oestrogen receptor content, determined biochemically, and histochemical localisation of the oestrogen binding components on thin sections of rat uteri, DMBA-induced mammary tumours and also human breast tumour tissue taken at mastectomy. The data indicate that in thin sections of tissue all of these conjugates appear to bind not to the classical oestrogen receptor moiety but to other oestrogen binding proteins.