Maf family transcription factors are important regulators in various differentiation systems. Putative Maf recognition elements (MAREs) are found in the 3′ enhancer region of the immunoglobulin heavy chain (IgH) gene. These elements are bound in B-cell extracts by a heterodimeric protein complex containing both Bach2 and a small Maf protein. Analysis of normal hematopoietic cells revealed that Bach2 is specifically expressed in B cells. Bach2 is abundantly expressed in the early stages of B-cell differentiation and turned off in terminally differentiated cells. Bach2 acts together with MafK as a negative effector of the IgH 3′ enhancer and binds to the co-repressor SMRT (silencing mediator of retinoid and thyroid receptor). Hence the Bach2–small-Maf heterodimer may represent the first example of a B-cell lineage, and of a developmental stage-restricted negative effector of the MARE in the IgH 3′ enhancer region.