The pre-B cell receptor (pre-BCR), composed of immunoglobulin mu heavy chain and the surrogate light chain (SLC) proteins lambda5 and Vpreb, signals for proliferation and maturation of developing pre-B cells. It has been assumed that pre-B cells stop cycling by the pre-BCR-mediated downregulation of SLC transcription. We generated transgenic mice expressing SLC throughout B cell development and, remarkably, found that enforced SLC expression had no effect on pre-B cell proliferation or differentiation. However, in the presence of conventional immunoglobulin light chains, SLC components had the capacity to induce constitutive BCR internalization, secondary immunoglobulin light-chain rearrangement, and a severe developmental arrest of immature B cells, dependent on the adaptor protein Slp65. Residual B cells in the spleen showed increased expression of surface CD5, which is a negative regulator of BCR signaling, and differentiated spontaneously into IgM+ plasma cells. Thus, the silencing of SLC genes is not essential for the limitation of pre-B cell proliferation, but is required for the prevention of constitutive activation of B cells.