The cis-acting elements that regulate production of germ-line transcripts from Ig heavy chain genes and subsequent switch recombination to those genes are poorly defined. We reported that a 17-kb transgene that includes Igamma1, Sgamma1, and Cgamma1 is regulated for germ-line transcription like the endogenous gamma1 gene. Transcripts from such transgenes are expressed only in B cells treated with both LPS and IL-4, and not in B cells treated with LPS alone or in thymocytes or nonlymphoid tissues. We have now found that transcripts from these transgenes are induced by treatment of transgenic B cells by IL-4 alone. As reported by others, IFN-gamma acts to inhibit the IL-4-mediated induction of germ-line transcripts of the endogenous gamma1 gene. We have found that LPS-plus IL-4-induced germ-line transcription of gamma1 transgenes is likewise inhibited by treatment of B cells with IFN-gamma, so the gamma1 gene must include the cis-acting element(s) that confers this inhibition. It is also known that CD40 ligation induces a modest amount of germ-line transcripts from the endogenous gamma1 gene and synergizes with IL-4 to induce large amounts of germ-line transcripts. The gamma1 transgenes are likewise induced by CD40 ligation, suggesting that the response element(s) for CD40 ligation can be found in the gamma1 gene. The promoter region for the germ-line transcripts and the I exon are likely to include some of these cis-acting elements. A series of transgenic mice with the promoter/Igamma1 region conferred low level, lymphoid-specific, RNA expression to a reporter gene, including significant expression in thymocytes. However, the promoter/Igamma1 transgenes were not regulated like the endogenous gamma1 gene, in that transcription in splenic B cells was not increased by LPS plus IL-4.