Primary cultures of bovine mammary secretory epithelial cells and spontaneously transformed mammary secretory epithelial cells were treated with estradiol 17-βor prolactin separately and in combination with both hormones. Both estradiol and prolactin induced BRCA1 expression in the cells. Hormone induced expression of BRCA1 was highest in transformed cells. Moreover, the induction of BRCA1 gene expression was highest when both estrogen and prolactin were in the culture medium. Gel shift assays showed that STAT5a antibody bound to a 20-bp double stranded oligonucleotide, containing a potential STAT5 response element (5`-acagtttctaaggaacactg-3`, 3`-tgtcaaagattccttgtgac-5`). This is the first report showing that bovine mammary cells, in culture, are responsive to estrogen and prolactin in inducing expression of bovine BRCA1 and that BRCA1 may contain a STAT5 response element directing expression of the bovine BRCA1 gene. Further work is needed to examine the presence of estrogen response element(s) in estradiol induced BRCA1 expression. One must determine if PRL induced BRCA1 induction occurs after transfection of STAT5 -/- cells with STAT5a and STAT5a mutant constructs. This will give the strongest evidence of the direct involvement of a STAT5–BRCA1 signaling pathway.