Immunoglobulin variable region promoters are predominantly B-cell specific, but the molecular basis for this specificity has not been elucidated. To further understand how B-cell-specific immunoglobulin promoter expression is mediated, the murine lymphoid cell line 2017 was engineered to express the green fluorescent protein under the control of an immunoglobulin heavy chain promoter and selected for high activity using multiple rounds of fluorescence-activated cell sorting. Rare clones with intense and stable immunoglobulin promoter activity were isolated. Transient transfection experiments demonstrated that two different immunoglobulin promoters and two other B-cell-specific promoters have higher activities in the selected cell lines relative to the parental line and to the non-cell-type-specific histone H2B promoter. The increased immunoglobulin activity required nucleotide residues downstream of the transcription initiation site which were also important for maximal activity in B cells and which were conserved in other B-cell-specific promoters. Unlike the unselected cells, the 2017 variants also showed activation of their endogenous immunoglobulin heavy chain variable regions.