The determination of enzyme activity or inhibition in intact living cells is a problem in the development of inhibitors for intracellular proteases. The production of fluorescent protoporphyrin IX (PpIX) from the nonfluorescent (N)-Gly/Pro-5-aminolevulinic acid (ALA) substrates was used to evaluate the prolyl/glycyl-specific dipeptidylpeptidase IV (DPPIV)-like and prolyloligopeptidase (POP)-like activities of human cells. The results demonstrated that whereas POP-like activity could be attributed to the actual POP, the DPPIV-like activity could be related to actual DPPIV only in one colon cell line. In the other breast and colon cell lines, DPPIV-like activity was intracellular and displayed by other prolyl-specific aminopeptidases. Our experiments also demonstrated the involvement of glycyl-specific proteases in the processing of ALA precursors. These observations have important consequences for the development and evaluation of selective inhibitors for these enzymes