the present invention relates to compound having the formula : ## str1 ## or a pharmaceutically acceptable acid salt thereof , wherein r is a polypeptide moiety comprises an amino acid sequence : - y - arg - pro - z - ile - leu ; wherein y is arg , and z is tyr or trp ; r 1 is a fluorophore selected from the group consisting of fluorescein , rhodamine , blue fluorescent and texas red ; and x is oxygen or sulfur . the present invention also relates to a method for the labeling of neurotensin receptors on cell surface and a method to isolate neurotensin - receptor expressing cells .