To delineate the role of the IgH 3' enhancer in the regulation of Ig heavy (IgH) chain gene expression, mice harbouring rearranged IgH transgenes, with (PSVμ3) and without (PSVμ1) this element, were produced. RNA and protein analysis from the different transgenic lines revealed a5-to 7-fold increase in the expression level of the transgene containing the IgH 3′ enhancer. This difference is also reflected at the protein level in hybridomas generated from the two transgenic lines. The elevation of transgene Ig expression in the PSVμ3 lines is restricted to activated B lymphocytes, an observation which is further supported by the ability of this transgene to be reactivated upon immunization. Interestingly, although the up-regulation of transgene expression In PSVμ3 animals is considerably higher in comparison to the PSVμ1 animals, a significant response is still observed in the PSVμ1 mice. We speculate therefore that the IgH locus is subject to transcriptional modification in late B cell development. Our data suggest that both the Eμ enhancer and the IgH 3′ enhancer can up-regulate transgene Ig expression, but the presence of the 3′ enhancer results in elevated levels of transgene Ig production. It therefore appears that the expression level of IgH genes is subject to transcriptional modification during B cell development. Additional control elements are most likely required for optimal Ig expression, since our expression data from the transgene in PSVμ3 animals are incompatible with endogenous Ig levels. The recent identification of additional enhancer elements in the far 3′ end of the IgH locus supports this possibility. The data presented here provides a sound basis for the production of high levels of mAb, possibly tailored to suit the needs of the researcher.