A tissue-specific enhancer (Eμ) lies between the joining (JH) and μ constant region (Cμ) gene segments of the immunogiobulin heavy chain (igH) locus. Since mouse endogenous igH genes are efficiently transcribed in its absence, the normal function of this enhancer remains ill-defined. Recently, another lymphold-specific enhancer of equal strength has been identified 3′ of the rat IgH locus. We have isolated an analogous sequence from mouse and have mapped it 12.5kb 3′ of the 3′-most constant region gene (Cα-membrane) of the BALB/c mouse locus. The mouse and rat sequences are 82% homologous and share with other enhancers several DNA sequence motifs capable of binding protein. However, In transient transfection assays, the mouse sequence behaves as a weaker enhancer. The role of this distant element in the expression of endogenous IgH genes, both In Eμ-deficient, ig producing cell lines and during normal B cell development, is discussed.