At the precursor B cell stage during bone marrow B cell development, Ig μH chain associates with surrogate L (SL) chain, which is encoded by the three genes VpreB1, VpreB2, and λ5, to form the pre-B cell receptor (pre-BCR). Surface expression of the pre-BCR is believed to signal both proliferation and allelic exclusion of the IgH locus. Mice which lack either VpreB1/VpreB2 or λ5 show a lack of precursor B cell expansion but normal IgH allelic exclusion. This would suggest that one of either λ5 or VpreB can make a pre-BCR-like complex which is still able to signal allelic exclusion but not proliferation. To investigate this, we established mice lacking all components of the SL chain. These mice showed severely impaired B cell development which was similar to that previously found in mice lacking either λ5 or VpreB1/VpreB2. Surprisingly, the IgH locus was still allelically excluded and thus the SL chain appears not to be involved in allelic exclusion.