cDNAs prepared using poly(A)mRNA from pituitaries and containing the coding sequences for bovine and porcine growth hormones (bGH and pGH) were cloned in bacteria. The primary structures of the peptide hormones derived from the nucleotide sequences of the respective cDNAs show approximately 90% homology. The cloned cDNAs were modified using synthetic DNA to construct expression vectors for efficient bacterial production of the mature animal growth hormones.