The expression of immunoglobulin heavy-chain (IgH) genes is generally thought to be regulated by the combination of the VH promoter with the enhancer element which is located in the JH-CH intron. This is probably an oversimplification: there are cell lines that transcribe IgH genes despite the deletion of the intron-enhancer. These findings could imply that other enhancer element(s) exist in the IgH locus. Here we show that a strong B-cell-specific enhancer is indeed located at the 3'-end of the rat IgH locus, 25 kilobases downstream of C alpha. This enhancer should be retained downstream of all rearranged IgH genes, regardless of the VH or CH segment used. Taken together with analogous findings for the mouse kappa locus, the results prompt a re-evaluation of the mechanism of regulation of immunoglobulin gene transcription. Furthermore, unlike the intron-enhancer, the IgH 3' enhancer would become linked to a c-myc that rearranges into an IgH switch region. The IgH 3' enhancer could therefore play a part in the activation of the translocated c-myc genes in rat immunocytomas, mouse plasmacytomas and Burkitt lymphomas.