The common acute lymphoblastic leukemia antigen (CALLA) is a 749-amino acid type II integral membrane protein expressed by most acute lymphoblastic leukemias, certain other lymphoid malignancies with an immature phenotype, and normal lymphoid progenitors. A computer search against the most recent GenBank release (no. 56) indicates that human CALLA cDNA encodes a protein nearly identical to the rat and rabbit neutral endopeptidase 24.11 ("enkephalinase;" EC 3.4.24.11). This zinc metalloendopeptidase, which has been shown to inactivate a variety of peptide hormones including enkephalin, chemotactic peptide, substance P, neurotensin, oxytocin, bradykinin, and angiotensins I and II, had not been identified in lymphoid cells. To determine whether CALLA cDNA derived from human acute lymphoblastic leukemia cells (Nalm-6 cell line) encodes functional neutral endopeptidase activity, we generated CALLA+ stable transfectants in the CALLA- murine myeloma cell line J558 and analyzed them for enzymatic activity in a fluorometric assay based upon cleavage of the substrate glutaryl-Ala-Ala-Phe 4-methoxy-2-naphthylamide at the Ala-Phe bond. Total lysates as well as whole-cell suspensions of the Nalm-6 line and of the CALLA+ transfectants, but not of the CALLA- J558 cells, possessed neutral endopeptidase activity. This enzymatic activity was associated with the cellular membrane fraction and was abrogated by the specific neutral endopeptidase inhibitor phosphoramidon. The unequivocal identification of CALLA as a functional neutral endopeptidase provides insight into its potential role in both normal and malignant lymphoid function.