Caseins are the major milk proteins in most mammals. Together with calcium and phosphate they form the casein micelle. The corresponding casein genes are clustered in mammalian genomes and their expression is coordinately regulated with regard to developmental and tissue specificity. Casein gene promoters are responsive to lactogenic hormones, cell-matrix, and cell-cell interactions. Transcriptional enhancer elements are found in the 5(') upstream regions of casein genes but have also been detected in the first intron of the bovine beta-casein gene. We show here that the first intron of the murine beta-casein gene has three discernible functions. First, transcriptional enhancer elements present in the intron increase the basal activity of the beta-casein promoter. In addition, these intronic enhancer elements augment the induction of the beta-casein promoter by lactogenic hormones. Finally, we demonstrate that the first intron of the murine beta-casein gene contains a functional promoter.