A region of the primary amino acid sequence of the epidermal growth factor receptor (EGF) protein‐tyrosine kinase, which is involved in ATP binding, was identified using chemical modification and immunological techniques. EGF receptor was 14C‐labelled with the ATP analogue 5′‐p‐fluorosulphonylbenzoyladenosine and from a tryptic digest a single radiolabelled peptide was isolated. The amino acid sequence was determined to be residues 716–724 and hence lysine residue 721 is located within the ATP‐binding site. Antisera were elicited in rabbits to a synthetic peptide identical to residues 716–727 of the EGF receptor and the homologous sequence in v‐erb B transforming protein from avian erythroblastosis virus. The affinity‐purified antibodies precipitated human ECF receptor from A431 cells and placenta, and the v‐erb B protein from erythroblasts. The antibodies inhibited EGF‐stimulated receptor protein‐tyrosine kinase autophosphorylation and phosphorylation of an exogenous peptide substrate containing tyrosine. The antibodies did not immunoprecipitate the transforming proteins pp60v‐src or P120gag‐abl or cAMP‐dependent protein kinase, proteins which have homologous but not identical sequences surrounding the lysine residue within the ATP‐binding site, nor did they react with the platelet‐derived growth factor receptor. The antibodies had no effect on the kinase activity of purified v‐abl protein in solution. The antibodies may therefore be a specific inhibitor of the tyrosine kinase of the EGF receptor.