X-linked agammaglobulinemia (XLA) appears to involve a defect in human B lymphocyte differentiation which is manifested at the pre-B cell stage. The defect segregates as an X-linked recessive trait but is not a single genetic entity. IgM-producing B cell clones were established by Epstein-Barr virus transformation of peripheral blood mononuclear cells of patients with the XLA defect linked to the DXS3 and DXS17 chromosomal loci. Individual XLA B cell clones were demonstrated to have rearrangements of the JH regions of both immunoglobulin VH region loci. The rearranged JH regions of the B cell clone ALA 19 were molecularly cloned and their nucleotide sequence was determined. Both JH-associated rearrangements (designated 191 and 192) resulted from the juxtaposition of variable (VH), diversity (D) and joining (JH) segments (VHDJH rearrangements). The 191 rearrangement employed a VH segment belonging to VH subgroup III and a JH4 segment. The 192 rearrangement employed a VHII and a JH6 segment. The D191 and D192 segments encompassed 21 and 28 nucleotides, respectively, and showed little homology to each other or to previously reported human D sequences. Surprisingly, both VHDJH complexes had open reading frames. However, in accord with principles of allelic exclusion, only the 191 allele was detectably expressed in the total RNA of the cell. A possible mechanism for the lack of expression of the 192 allele is discussed. We conclude that the DXS3-DXS17-linked XLA defect does not preclude VH to DJH rearrangements or the expression of VH containing heavy chain molecules.