Regulation of immunoglobulin heavy chain (IgH) gene expression is controlled by a B cell-specific promoter, intronic enhancer and additional B cell-specific eleancer elements identified recently in the 3 end of the IgH locus. One of the latter elements, the IgH 3 enhancer, is of particular interest: (1) it is B cell-specific and active only in late B cell development; (2) in rodent plasmacytomas and in some human Burkitt's lymphomas it is part of a locus control region (LCR) that is involved in deregulation of the c-myc oncogene as a result of translocation into the IgH locus; and (3) it has been implicated in the mechanisms that control Ig gene class switch recombination. We have used a somatic cell hybridization approach to genetically analyse regulation of the activity of the IgH 3 enhancer. When mouse MPC11 plasmacytoma cells, in which the IgH 3 enhancer is active, are fused with fibroblasts, Ig expression is extinguished at the level of transcription. Here we show that in a MPC11 plasmacytoma fibroblast environment, the IgH 3 enhancer is transcriptionally inactive. Furthermore, we demonstrate that binding of several B cell-specific transcription factors, essential for IgH 3 enhancer activity, is lacking, which may explain 3 enhancer inactivity, although the binding of repressors cannot be excluded. Moreover, the high expression level of c-myc, characteristic of the parental MPC11 cells carrying the t(12;15) translocation, is down-regulated in the hybrids to that in unfused fibroblasts. Therefore, inactivation of the IgH 3 enhancer is a multifactorial process affecting several transcription factors that control the cell-specific and developmental activity of the enhancer.