BP-1, also known as Ly-51/6C3 antigen, is a type II homodimeric transmembrane glycoprotein formed by two disulfide-linked chains of 140 kD.1, 2 Originally identified on the surface of pre-B cell lymphomas1, 3 the cDNA encoding BP-1/Ly-51/6C3 has been cloned and found to represent aminopeptidase A.4, 5 It is expressed at high levels on bone marrow stromal cell lines which support in vitro B lymphopoiesis and on a wide variety of mouse tissues known to possess aminopeptidase activity.6-8 Subsets of normal bone marrow pre-B and B cells express low levels of BP-1/Ly-51/6C3, which is rapidly upregulated on pre-B cells in the presence of IL- 7.9, 10 The BP-1/6C3 marker is commonly used to identify B cells in different stages of development.11