The antigen defined by the cluster designation group 10 (CD 10) has also been called common acute lymphoblastic leukemia antigen (CALLA) and neutral endopeptidase (NEP). This antigen is expressed by a variety of normal cell types, including lymphoid precursor cells, germinal center B lymphocytes, and some epithelial cells. On the cell surface, CD10 is an enzyme that functions to reduce the cellular response to peptide hormones. On neoplastic cells, the antigen is detected in a high percentage of cases of acute lymphoblastic leukemia (ALL), follicular lymphoma, Burkitt's lymphoma, and some nonhematopoietic tumors. CD10 is not a lineage-specific antigen, but its detection may be of value in combination with other antibodies. The detection of this antigen is most valuable in the diagnosis and classification of lymphoblastic malignancies, follicular lymphomas, and Burkitt's lymphoma.