The present invention provides an active binding sequence of mammalian alpha- fetoprotein (AFP) to the receptor of AFP (AFPr or RECAP). The sequence is embodied in peptides comprising Lys-Glx-Glx-Xaa-Leu-Ile-Asn (SEQ. ID. NO: 1) and variants thereof, wherein GIx means GIn or GIu, each GIx being selected independently of the other, and Xaa represents Phe or Leu. The peptides bind a site of the AFP receptor. This peptide can be used as a substitute for AFP in the detection, purification and imagining of RECAF. This peptide, as it binds to RECAF which is elevated in cancer cells, allows for a method of diagnostic determination of cancer or chemotherapeutic delivery using cytotoxic or radiological agents.
