Here we describe the reactivity of monoclonal antibody (mAb) ER-BMDM1, directed against a 160-kDa cell membrane-associated antigen (Ag) with amino-peptidase activity. The aminopeptidase recognized by ER-BMDM1 is present on various mouse macrophage (M phi) and dendritic cell (DC) subpopulations as well as on microvillous epithelia. Analysis of ER-BMDM1 Ag expression in in vitro models of M phi maturation revealed that the Ag is expressed at increasing levels upon maturation of M phi. In vivo, high level expression of the ER-BMDM1 Ag occurs after the monocytic stage of maturation, since bone marrow cells and peripheral blood monocytes are essentially ER-BMDM1 negative. Analysis of isolated-resident and elicited M phi populations showed that ER-BMDM1 recognizes a specific subpopulation of mature M phi: only some resident peritoneal and alveolar M phi are ER-BMDM1 positive, whereas virtually all thioglycollate-elicited peritoneal exudate M phi bind the mAb. In lymphoid organs, a subpopulation of M phi is recognized as well as interdigitating cells (IDC) located in T cell areas. Phenotypic analysis of isolated DC--the in vitro equivalents of IDC--from spleen and lymph nodes confirmed that the majority of this important antigen-presenting cell population expresses the ER-BMDM1 aminopeptidase. The molecular characteristics of the ER-BMDM1 Ag suggest that it may represent the mouse homolog of human CD13.