11c-labelled peptide for detecting a tumour which expresses a peptide transporter

The use of a peptide (1) for producing an agent for detecting a tumour (18) which expresses a peptide transporter (PEPT) (4) is described. The peptide binds (1) to the pepetide transporter (PEPT) (4) and has an 11 C carbon atom on the carbonyl carbon of the N-terminal amino acid (3). Also described is a radiopharmacon for localizing a tumour (18) which expresses a peptide transporter (PEPT) (4). This comprises a peptide (1) which binds to the peptide transporter (PEPT) (4) and has an 11 C carbon atom on the carbonyl carbon of the N-terminal amino acid (3).