Mature, resting rodent, and primate B lymphocytes express two membrane Ig isotypes, IgM and IgD. Although membrane IgD production by these cells is regulated at a transcriptional level, and does not require deletion of the Cμ gene, Cμ has been deleted in all of the IgD-secreting tumor cells that have been studied. These IgD-secreting tumors, which include two mineral oil-induced plasmacytomas and three IgD-switch variants of an IgM-secreting hybridoma, might not, however, be representative of the rare IgD-secreting cells generated in response to an immune stimulus. A recent study of mice injected with a goat antibody to mouse IgD has demonstrated the generation of a relatively large secretory IgD response in these animals. We have now produced hybridomas by fusing spleen cells from these mice with a non-Ig-secreting plasmacytoma. Two of these hybridomas, KWD-1 and KWD-2, secrete IgD and express cell membrane IgD. Both of these hybridomas were found to have deleted the Cμ gene. KWD-2 produces a δ-chain mRNA and a δ-chain protein similar in size to those previously reported for normal secreted mouse IgD; however, KWD-1 synthesizes a secretory δ-chain mRNA that is approximately 0.25 kb smaller than the KWD-2 secretory δ-chain mRNA and secretes IgD with a δ-chain that is approximately 21 kDa smaller than the secretory δ-chain of KWD-2. ELISA studies with epitope-defined anti-δ mAb indicate that KWD-2 has both δFc (Cδ1) and δFd (Cδ3) determinants, whereas KWD-1 has δFc but not δFd. These studies also demonstrate that the Ag-binding site of KWD-1 is not deleted because KWD-1 specifically binds goat IgG. Northern blot analyses with exon-specific probes indicate that while both KWD-1 and KWD-2 synthesize κ-chain mRNA and δ-chain mRNA that includes the VH, Cδhinge, and Cδ3 exons, the Cδ1 exon is present only on the KWD-2 δ-chain mRNA. Southern blot analysis confirms that the Cδ1 exon has been deleted in KWD-1, but not KWD-2. We have previously noted that a secretory δ-chain mRNA that is similar in size to that produced by KWD-1 accounts for approximately 25% of the splenic secretory δ-chain mRNA produced by goat anti-mouse IgD antibody-injected mice. Northern blot analysis of splenic mRNA from these mice indicates that this small δ-chain mRNA, such as that produced by KWD-1, lacks the Cδ1 exon. These results indicate that 1) the generation of an IgD-secreting cell during an immune response is accompanied by deletion of the Cμ gene; 2) deletion of Cδ1 is a relatively common event in the generation of IgD-secreting cells; and 3) the Cδ1 domain is not required for the expression of cell membrane IgD, the secretion of IgD, or the expression of antibody activity by IgD.