We have established the exon-intron structure of the gene coding for the constant (C) region of the mouse immunoglobulin delta heavy chain, using DNA clones isolated from BALB/c embryos and the delta mRNA extracted from two delta-producing hybridomas, B1-8. delta 1 and GCL2.8. At least three types of C delta gene structures are identified. A 2.7 kb delta mRNA reveals six exons. This delta mRNA may code for a membrane-bound delta chain. A second delta mRNA of 1.8 kb shares the first (5' side relative to direction of transcription) three exons with the 2.7 kb delta mRNA and in addition contains a fourth exon unique to this mRNA species. This delta mRNA most likely codes for a secreted delta chain. A third delta mRNA, also of 1.8 kb, shares the first four exons and a part of the fifth exon with the 2.7 kb mRNA. Its function, if any, remains unclear. We investigated the question of how a lymphocyte can produce the mu and delta heavy chains simultaneously, using the hybridoma GCL 2.8, which makes both IgM and IgD. Results of Southern gel blot analysis and gene cloning experiments indicate that this cell utilizes the same rearranged VH gene for the synthesis of the mu and delta chains, and yet maintains the embryonic configuration for the C mu and C delta genes and for the intervening region. Based on these results, we conclude that the VH sequence is spliced alternatively to the C mu or C delta sequence during processing of the primary RNA transcript. An alternative mechanism for the expression of the delta gene is found in hybridoma B1-8. delta 1, which actively secretes delta chains and synthesizes no mu chain. This mechanism involves deletion of the C mu gene, which brings the complete VH gene closer to the C delta gene.