Many of the cell fate decisions in precursor B cells and more mature B cells are controlled by membrane immunoglobulin (Ig) M heavy chain (m mu) and the Ig alpha-Ig beta signal transducers. The role of Ig beta in regulating early B cell development was examined in mice that lack Ig beta (Ig beta(-/-)). These mice had a complete block in B cell development at the immature CD43(+)B220(+) stage. Immunoglobulin heavy chain diversity (D-H) and joining (J(H)) segments rearranged, but variable (V-H) to DJ(H) recombination and Immunoglobulin messenger RNA expression were compromised. These experiments define an unexpected, early requirement for Ig beta to produce B cells that can complete VDJ(H) recombination.