Human gastrin has been shown to consist of two linear peptides, the heptadecapeptide amide [graphic omitted] (R = H) and its sulphate ester (R = SO3H). These are [Leu5]-analogues of the porcine hormones. The structures were deduced from the results of amino-acid analysis and enzymatic degradation, and confirmed by synthesis of the unsulphated peptide. Experiments on the conversion of this peptide into its sulphate ester are described.