W e established a transformed B cell line expressing both IgM and IgG on the cell surface from a patient with hyper IgM immunodeficiency using Epstein-Barr viruses. DNA and RNA of the cells were analyzed. DNA rearrangements of Ig JH gene loci were observed on both chromosomes. Cloning and DNA sequence analyses showed that one has a VHDHJH structure while the other has a DHJH structure. Southern hybridization with 5'-S mu and S gamma region-containing probes indicated germline configuration in the switch regions of mu and gamma genes on both chromosomes. To test expression of mu and gamma chains in the transformed cells at the mRNA-level, we used the polymerase chain reaction with three kinds of synthetic oligonucleotides as primers, one of which was part of the VH gene, while the other two were complementary to parts of C mu and C gamma genes. Sequence analysis of the amplified products showed that the same VHDHJH sequence is directly connected with either the C mu or the C gamma sequence in the mRNAs. This is direct evidence showing that in double isotype-bearing cells one VHDHJH exon in the transcript is alternatively spliced to C mu or C gamma without DNA rearrangement. The defect in this disease could be at the S-S recombination stage.