kappa-Casein gene promoter was localized within a 570-bp fragment (-552/+18) of a 5'-flanking region by the gene transfection assay. Deletion mutation analysis in mammary epithelial cell line, HC11, suggested that there are regulatory element in a region from -439 through -125. Some nuclear proteins from lactating rate mammary gland bind to this region specifically. One of them expressed preferentially during pregnancy bound to a 132-bp fragment (-439/-308) and another expressed preferentially during lactation bound to a 183-bp fragment (-307/-125).