The VH4-34 (VH4.21) gene has been repeatedly found to encode monoclonal anti-i/l cold agglutinins and is occasionally used by other autoantibodies with anti-DNA and rheumatoid factor specificity. To understand the basis for the frequent expression of the VH4-34 gene in autoimmunity, we prepared linker-based, amplified C mu and C gamma V gene (cDNA) libraries from the peripheral blood of two healthy adults (PBL-3 and PBL-4). The frequency of VH4 family gene-containing clones was determined with VH family-specific oligonucleotide probes, and by random sequencing we examined the frequency of VH4-34 gene expression among the VH4+ C mu clones. Collectively, these studies suggested that VH4-34 gene expression accounted for 7.8 and 10.3% of all IgM clones in the cDNA libraries of PBL-3 and PBL-4, respectively. In VH4 family-specific libraries prepared from genomic rearranged Ig DNA, the VH4-34 gene was also overrepresented relative to other VH4 family genes. In both the mu-cDNA as well as the rearranged Ig DNA libraries, the VH4-34-expressing clones were 95 to 100% homologous to the germline sequence and contained different CDR3 sequences. By contrast, the VH4-34-expressing C gamma clones showed somatic diversification from germline; in addition, the C gamma clones often had the same CDR3 sequences, suggesting that these clones were derived from either activated B cells or expanded, clonally related B cells. Multiparameter flow cytometric analysis of peripheral blood lymphocytes indicated that 3.2 to 6.2% of mu+CD20+ cells express the VH4-34-related Id 9G4. These current findings are relevant to the interpretation of restricted Ig gene usage reported in monoclonal cold agglutinin disease and other autoimmune disorders.