B lymphocyte differentiation is characterized by an ordered series of Ig gene assembly and expression events. In the majority of normal B cells, assembly and expression of Ig heavy (H) chain genes precedes that of light (L) chain genes. To determine the role of the Ig heavy chain protein in B cell development and L chain gene rearrangement, we have generated mice that cannot assemble Ig H chain genes as a result of targeted deletion of the J_H gene segments in embryonic stem cells. Mice homozygous for this deletion are devoid of slg⁺ B cells in the bone marrow and periphery. B cell differentiation in these mice is blocked at the large, CD43⁺ precursor stage. However, these precursor B cells do assemble ϰ L chain genes at a low level in the absence of μ H chain proteins. These data demonstrate that rearrangement and expression of the μ H chain gene is not absolutely required for ϰ L chain gene rearrangement in vivo. Expression of μ chains may facilitate either efficient L chain gene rearrangement or the survival of cells that have rearranged light chain genes by promoting the differentiation of large, CD43⁺ to small, CD43⁻ pre-B cells.