A novel nonapeptide with neurotensin-like immunoreactivity was isolated from pepsin-treated human plasma by dialysis, ion-exchange chromatography and high performance reversed-phase liquid chromatography. The amino acid sequence was determined by automated gas-phase sequence analysis as Ile-Ala-Arg-Arg-His-Pro-Tyr-Phe-Leu. Sequence homology with human serum albumin and with the biologically active peptides neurotensin and angiotensin is demonstrated. The name proposed for this peptide is kinetensin.