1. The literature concerning the mechanism of action of steroid hormones, has been reviewed. In particular that relating to steroid receptor proteins and their interaction with the cell nucleus and nuclear material has been stressed. 2. Nuclei have been isolated from human myometrium and their purity assessed using standard criteria. The isolated nuclei have been extracted using either 2M NaCl or 0.1M H2SO4. The resultant extracts have been immobilised on sepharose 4B and assayed for the ability to specifically bind activated, labelled oestrogen receptor complexes. 3. Both nuclear fractions have shown significant ability to specifically bind activated, labelled oestrogen receptor complexes. 4. The relative abilities of oestrogen receptor from a variety of sources to bind to the 2M NaCl soluble nuclear extract/sepharose resin has been assessed. Receptor from immature rat uterus was found to bind in a reproducible, saturable manner to the myometrial nuclear extract and was used as the source of oestrogen receptor for all subsequent studies. 5. The effect of assay conditions on the binding activity at
