The monoclonal antibody AG6 was raised against HUVE cells and was confirmed as recognizing CD13 (aminopeptidase N) by its reactivity with the CD13 cDNA transfectant pipz "4". However, in sequential immunoprecipitation studies with another anti-CD13 monoclonal, WM15, neither was able to completely remove the molecules recognized by the other. As both monoclonals recognize the same cDNA transfectant, the differences cannot be in the amino acid sequence, suggesting that the CD13 subpopulations have glycosylation differences. Neither neuraminidase and endoglycosidase H treatment of the immunoprecipitates, nor tunicamycin and monensin treatment of proliferating cells differentiated the molecular subpopulations. The epitopes are protein and not carbohydrate as both monoclonals were able to precipitate deglycosylated CD13 from tunicamycin- and monensin-treated cells and o-glycanase-treated purified CD13. Sequential immunoprecipitation studies with WM15 and AG6 of pipz "4" and purified o-glycanase-treated CD13 showed that the subpopulations were due to the O-linked oligosaccharides. Sequential immunoprecipitations with seven other CD13 monoclonals show that there are at least five subpopulations of CD13. It is postulated that the differences between the subpopulations are due to differential utilization of glycosylation sites or subtle differences in oligosaccharide composition causing variable masking of protein epitopes. Similar observations with the integrin group of molecules suggests that this may be an example of a more general phenomenon among glycoproteins.