For the investigation of the possibility of its being a marker enzyme for tumor cells, the activity of dipeptidyl peptidase (DPP) IV (EC 3.4.14.5), a membrane-bound enzyme, in cultured human carcinoma cells was examined. The homogenates of three carcinoma cell lines (HeLa, KB, and K-44) contained lower glycylprolyl methylcoumarinamide (Gly-Pro-MCA) hydrolase activities at pH 8.7 (assumed to be DPP IV) and higher activities of alkaline phosphatase and gamma-glutamyl transpeptidase, which are also membrane-bound enzymes, than those of normal human fibroblasts (HF). Examination of carcinoma cells for the subcellular localization and pH optimum of Gly-Pro-MCA hydrolase activity revealed that the activity of a lysosomal enzyme that hydrolyzes Gly-Pro-MCA at pH 6.4 was markedly increased in carcinoma cells, but not in normal cells. The separation and characterization of Gly-Pro-MCA hydrolases by gel filtration, affinity chromatography, and substrate specificity demonstrated that HF have three peaks indicating DPP IV, DPP II, and an unknown enzyme, whereas the three carcinoma cell lines gave a prominent peak indicating DPP II and a trace of DPP IV. The DPP II activity was 6- to 24-fold higher in carcinoma cell lines than in HF, and it also was 2.85- to 4.13-fold higher than the DPP IV activity in carcinoma cell lines but was 10-fold lower in HF. These clear enzymatic differences between carcinoma cells and normal HF may be useful as a marker of malignancy.