The variable (V) regions of C108G, a potent neutralizing chimpanzee mAb against a glycan-dependent epitope in the V2 region of HIV-1 gp120, have been characterized for reactivity with human VH and VK family-specific antisera, and their nucleotide sequences have been determined and analysed. To our knowledge, this is the first study characterizing expressed chimpanzee VH and VK genes. Results show that C108G expresses members of the VH3 and VK1 families, the largest VH and VK families in humans, respectively. Nucleotide and amino acid sequence analyses reveal that C108G VH is most homologous to the human VH3 germline gene, hsigdp33 or V3-43, and the human JH4 minigene. The human germline VK1 gene that is most homologous to C108G VK, hsigk1012, was previously observed in unmutated form in a human autoantibody with anti-i red blood cell antigen specificity and in seven human Fabs and a mAb directed against epitopes overlapping the CD4-binding site of HIV-1 gp120. This germline gene was unmutated in three of the human Fabs and was somatically mutated in the other four Fabs and the mAb. In addition, the JK minigene was used in C108G VK, JK2, is apparently over-represented in anti-HIV-1 mAbs/Fabs; this minigene was used in 61% of the anti-gp120 human Fabs recently described and in three other anti-CD4-binding site human mAbs derived by EBV transformation. While the significance of these findings is unclear, they may suggest a bias in VK/JK gene usage and/or network regulation involving an hsigk1012/JK2 idiotope(s) in the antibody response to HIV-1. Both the C108G VH and VK genes showed evidence of somatic mutation and antigen selection that apparently occurred in vivo during chronic exposure to HIV-1 and its antigens. Surprisingly, this somatic mutation was most profound in the CDR3 region of C108G VK; this region shared only 48% nucleotide homology with hsigk1012 contrasted with a homology of 94% over the remainder of these two V gene sequences. Perhaps the most significant finding of this study is that the expressed VH and VK genes of chimpanzee mAb C108G are no more divergent from their most homologous human germline genes than are the expressed V genes of several recently characterized human anti-HIV-1 mAbs/Fabs from their apparent human germline genes. This suggests that chimpanzee mAbs are no more likely to elicit deleterious anti-immunoglobulin responses in humans than are human mAbs and emphasizes the potential for development of chimpanzee mAbs as immunotherapeutic agents.