The development of B lymphocytes from progenitor cells is dependent on the expression of a pre-B cell-specific receptor made up by a μ heavy chain associated with the surrogate light chains, immunoglobulin (Ig)α, and Igβ. A variant pre-B cell receptor can be formed in which the μ heavy chain is exchanged for a truncated μ chain denoted Dμ. To investigate the role of this receptor in the development of B cells, we have generated transgenic mice that express the Dμ protein in cells of the B lineage. Analysis of these mice reveal that Dμ expression leads to a partial block in B cell development at the early pre-B cell stage, probably by inhibiting VH to DHJH rearrangement. Furthermore, we provide evidence that Dμ induces VL to JL rearrangements.