The complete amino acid sequence of human .beta.-endorphin was obtained by automatic sequencing of a sulfonyl isothiocyanate derivative of this peptide, in combination with peptide mapping of a tryptic digest of the native molecule. It was identical with the carboxy-terminal portion 61-91 of human .beta.-lipotropin (.beta.-LPH). The morphine-like activity of .beta.-endorphin is comparable both in the mouse vas deferens bioassay and in the opiate receptor binding assay. However, .beta.-LPH is not active up to concentrations of 10-6M.