Many of the cell fate decisions in precursor B cells and more mature B cells are controlled by membrane immunoglobulin (Ig) M heavy chain (mμ) and the Igα-Igβ signal transducers. The role of Igβ in regulating early B cell development was examined in mice that lack Igβ (Igβ−/−). These mice had a complete block in B cell development at the immature CD43+B220+ stage. Immunoglobulin heavy chain diversity (DH) and joining (JH) segments rearranged, but variable (VH) to DJH recombination and immunoglobulin messenger RNA expression were compromised. These experiments define an unexpected, early requirement for Igβ to produce B cells that can complete VDJH recombination.