The complete amino acid sequence of growth hormone (GH) from a reptilian species (the sea turtle, Chelonia mydas) has been determined for the first time. The hormone was reduced, carboxymethylated, and subsequently cleaved in turn with cyanogen bromide and Staphylococcus aureus protease. The intact protein was also cleaved with lysyl endopeptidase and o-iodosobenzoic acid. The resulting fragments were exclusively separated by reversed-phase high-performance liquid chromatography and subjected to sequence analysis by automated gas-phase sequencer employing the Edman method. The sea turtle GH consist of 190 amino acid residues with two disulfide linkages formed between residues 52-160 and 180-188, and possesses a microheterogeneity, indicated by the presence or absence of an additional alanine residue at the N-terminus. Sequence identities of sea turtle GH to other species of GH are 89% with chicken GH, 79% with rat GH, 68% with blue shark GH, 58% with eel GH, 59% with human GH, and 40% with a teleostean GH such as chum salmon. On the basis of amino acid sequence comparisons, a molecular phylogenetic tree is proposed.