A chicken oviduct cDNA clone containing the complete open reading frame of the oestrogen receptor (ER) has been isolated and sequenced. The mol. wt of the predicted 589-amino acid protein is approximately 66 kd which is very close to that of the human ER. Comparison of the human and chicken amino acid sequences shows that 80% of their amino acids are identical. There are three highly conserved regions; the second and third of which probably represent the DNA- and hormone-binding domains of the receptor. The putative DNA-binding domain is characterised by its high cysteine and basic amino acid content, and the hormone-binding domain by its overall hydrophobicity. These two domains of homology are also present in the human glucocorticoid receptor (GR) and the product of the avian erythroblastosis virus (AEV) gene, v-erbA, indicating that c-erbA, the cellular counterpart of v-erbA, belongs to a multigene family of transcriptional regulatory proteins which bind steroid-related ligands. The first highly conserved ER region is not present in the truncated v-erbA gene, but shares some homology with the N-terminal end of the GR. The function of the v-erbA gene product is discussed in relation to its homology with the ER and GR sequences.