Ig protein and mRNA expression was examined in a collection of 18 monoclonal EBV-transformed B cell lines derived from five patients with X-linked agammaglobulinemia (XLA). A diversity of H and L chain isotypes were synthesized by these lines: the majority (12 lines) expressed mu kappa chains, while mu lambda (two lines), gamma kappa (one), gamma lambda (one), delta lambda (one), and alpha kappa (one) isotype expression was also observed. For all the mu kappa-producing XLA B cell lines, the mu and kappa mRNA transcripts were of native size, and sequence analysis across the regions of VHDJH and V kappa J kappa gene joining showed that Ig gene rearrangements occurred in a typical manner. A variety of VHDJH and V kappa J kappa gene rearrangements were observed, not only within the set of mu kappa+ XLA B cells as a whole, but also among the cell lines derived from single patients. Southern blot analysis for genomic Ig H chain gene rearrangements was done to fully assess the extent of clonal heterogeneity among multiple mu kappa+ XLA B cell lines derived from two patients; all the B cell lines possessed distinct gene rearrangement patterns demonstrating their clonal unrelatedness. Our findings indicate that the B cell repertoire in individual XLA patients is clonally diverse and that it is unlikely that the defect in B cell differentiation in XLA is the result of inefficient or ineffective rearrangement of Ig H or L chain genes. Rather, this study provides support for the idea that the XLA defect relates to a more generalized cellular function, such as regulating the proliferation and/or clonal expansion of cells of the B lymphoid lineage.