A cDNA of 1065 bp encoding the human milk beta-casein was cloned and sequenced using a synthetic oligodeoxyribonucleotide probe and a human mammary gland library. The nucleotide (nt) sequence contained an open reading frame sufficient to encode the entire amino-acid (aa) sequence of a beta-casein precursor protein consisting of 210 aa and a signal peptide of 15 aa. The nt sequence shows 45-62% homology to those of bovine, ovine, rat, and mouse beta-caseins. The highly phosphorylated site, which is responsible for the calcium-binding capacity of beta-casein, the signal peptide, and a sequence encoding for an inhibitor to the angiotensin-converting enzyme seem highly conserved among the beta-caseins with known sequences.