Mouse mammary tumour virus (MMTV) gene expression has been shown to be regulated by glucocorticoids. A hormone response element (HRE) located between -202 and -59 upstream of the start of transcription in the long terminal repeat (LTR) region of the proviral DNA is required for this induction. We have investigated the role played by the HRE in the induction of MMTV LTR transcription by other classes of steroid hormones. Chimaeric constructs containing the HRE and the authentic LTR promoter linked to an indicator gene or the HRE linked to an otherwise hormone insensitive promoter directing the transcription of an indicator gene, were transfected into the human mammary tumour cell line T47D. Transcription at the MMTV LTR promoter or at the previously hormone-insensitive promoter was induced by progestins and androgens but not by oestradiol in transfected cells that contained functional receptors for these hormones. These results identify the HRE as the cis-acting element that mediates the progestin and androgen induction of MMTV LTR transcription. The HRE is therefore a DNA element that is required not just for glucocorticoid but also for progesterone and androgen induction of MMTV LTR transcription.