A triazole containing pentapeptide labeled with the positron-emitting isotope fluorine F 18, used as a tracer for positron emitting tomography (PET) imaging. As a caspase-3 specific substrate, fluorine F 18 CP18 peptide is preferentially taken up by and accumulates in tumor cells with high caspase-3 activity. Upon PET imaging, apoptotic cells can be detected and apoptotic activity can be assessed. Caspase-3 is a cysteine protease that plays a key role in the induction of apoptosis.