To examine the DNA sequences involved in regulation of kappa immunoglobulin gene expression, an assay was developed whereby transcription of a cloned, rearranged kappa gene could be detected following its transfection into antibody-secreting mouse myeloma cells. In a transient expression assay, RNA transcripts initiating 20 to 25 bp before the AUG translation initiator of the kappa gene were readily detected. The constant region (C kappa) and part of the J kappa-C kappa intron was then deleted from the rearranged gene and a downstream SV40 polyadenylylation signal was provided. No transcription was detected from the deleted gene upon transfection into myeloma cells, suggesting that after rearrangement the kappa variable region promoter is activated by sequences more than 2.6 kb downstream of J kappa. The rearranged kappa gene was also transfected into a nonlymphoid line of monkey cells and RNA transcripts were detected. Transcription in these cells, however, was at a relatively lower level than in myeloma cells, did not depend on the presence of downstream sequences, and initiated from a site about 10 bp downstream of the myeloma site. These experiments define a sequence downstream of the promoter that is necessary for accurate transcription of the kappa chain gene.