Previous studies have shown that the human polyomavirus BK (BKV) genome contains an oestrogen response element (ERE). This isolated element binds its cognate receptor in vitro and can mediate 17beta-oestradiol-induced gene expression when linked to a heterologous promoter. The roles of the ERE- and the AP-1-binding sites in oestrogen receptor-directed transcription from the complete BKV promoter/enhancer (Dunlop strain) have been examined and the effects of the general co-activator CBP and large T- and small t-antigens on oestrogen receptor-mediated transcription have been investigated. A constitutive activated oestrogen receptor stimulated BKV promoter activity in HeLa cells. Mutations in either the ERE- or the AP-1-binding sites did not impair oestrogen receptor-induced activation of the BKV Dunlop promoter, while mutations in both binding motifs almost completely abolished oestrogen receptor-induced transcription. Simultaneous expression of large T- and small t-antigens strongly activated oestrogen receptor-mediated transcription. When expressed separately, only large T-antigen moderately stimulated oestrogen receptor-mediated transcription. The stimulatory effect of large T-antigen on the activity of the oestrogen receptor is probably indirect because no physical interaction between the two proteins was detected in a two-hybrid assay. Large T-antigen abrogated the synergistic effect on transcription between this nuclear receptor and the general co-activator CBP. The findings that the BKV early proteins amplify oestrogen receptor-mediated transcription may have important biological implications in individuals with raised oestrogen concentrations.