Recently we identified an additional enhancer in the 3' end of the immunoglobulin heavy chain (IgH) locus. To identify individual regulatory elements within the rat IgH 3' enhancer, deletion analysis was performed. Transfection experiments using reporter constructs suggest that the enhancer contains three functionally distinct domains, two of which are lymphoid specific and one domain is active in both lymphoid and in nonlymphoid cells. The three domains together contribute to enhancer function and act synergistically. Further analyses suggest that a putative mu E1 site, octanucleotide motif, mu E3 site, and mu B/Ets-like motif are important for the overall transcriptional activity of the IgH 3' enhancer. Moreover, we provide evidence that an additional Ets-like element, micro A, is involved in the tissue specific regulation of enhancer activity and that binding of a protein to this element correlates with the transcriptional activity of one of the lymphoid restricted domains.