Southern blot-hybridization analysis of 10 breast carcinoma cell lines demonstrated an amplification of the epidermal growth factor (EGF) receptor gene in the BT20 cell line. The gene was amplified 10- to 14-fold when compared to the level seen in human placental DNA. Northern blot analysis indicated an increased level of transcription in this cell line which probably accounts for the high level of the EGF receptor reported in the BT20 cell line.