Using a transient co-transfection system we have identified a functional oestrogen responsive element (ERE) in the near vicinity of the TATA box of the chicken ovalbumin gene promoter. Oestrogen induced activation of transcription was observed only in chicken embryo fibroblast cells, and not in human HeLa cells, suggesting the existence of a cell-specific transcription factor(s) that is necessary for the hormonal response. By 5'-deletion mapping and site-directed mutagenesis, a sequence encompassing the more proximal of two GGTCA half-palindromic motifs was identified as a functional ovalbumin gene ERE. Both motifs correspond to oestrogen receptor binding sites in vitro as revealed by DNase I footprinting. Optimal response to the hormonal stimulus apparently requires stereoalignment of the ERE and the TATA box.