B cells and their antibody-secreting progeny represent one of several differentiation pathways that haematopoietic stem cells (HSC) may enter^1,2. Cells representing intermediate stages between HSC and B cells have been identified in mammalian haematopoietic tissues^3,4 and studied intensively over the past decade. This population of early B-lineage cells, termed pre-B, is characterized by cellular proliferation and an orderly cascade of immunoglobulin gene rearrangements^5–7, a combination of events leading to the generation of clonally diverse B cells which then migrate to peripheral lymphoid tissues. It remains to be determined what elements determine the polyclonal growth of pre-B cells, how immunoglobulin gene rearrangements are regulated, and what happens to pre-B cells undergoing ‘non-productive’ immunoglobulin gene rearrangements. These issues could be resolved more easily if early B-lineage cells could be identified precisely and isolated. Here, we describe a cell surface glycoprotein that is selectively expressed by pre-B and newly formed B cells in murine haematopoietic tissues. The molecule, a homodimer formed by disulphide-linked chains of relative molecular mass (M_r) 140,000, is identified by a mouse monoclonal alloantibody called BP-1.