The !'i-casein gene is a member of a small gene family encoding the calcium-sensitive caseins, which are specifically synthesized and secreted by the mammary gland during lactation in response to both peptide and steroid hormones. The caseins are involved in the transport of calcium phosphate in milk, which is important for bone development in the infant mammal. We report here the organization and complete DNA sequence of the 8·5 kb long bovine !'i-casein gene. Comparison with the rat !'i-casein gene reveals that the exons of both genes correspond exactly. The 5' flanking sequences of all Ca-sensitive casein genes are conserved within the proximal 200 bp and contain several elements that probably function as cis-acting regulatory elements, including an octamer-like motif, an SV40-type core enhancer and a sequence that appears to be common to all lactoprotein genes. The latter sequence is flanked on either side by 12 bp direct repeats. These direct repeats are themselves each part of sequences that display two-fold symmetry. The first 30 nucleotides of the 3' flanking regions in the bovine and rat !'i-caseins are well conserved, indicating that they are likely to be involved in the mechanism of 3' end processing of the primary transcript.