The sequences of 34 swine H chain V-regions expressed with either IgG, IgA, or IgM C regions in adult swine and newborn piglets are described and compared. Sixteen of these V-regions were cloned by using a specific leader primer whereas 18 were cloned by anchored PCR. According to the operational criterion of VH family classification, i.e., > 80% DNA sequence similarity, all sequences belong to a homogeneous VH gene family. A total of 31 additional VH-bearing C mu clones obtained by anchored PCR from colostrum-deprived newborn piglets hybridized equally with a probe for C mu and a pan-VH probe. When the consensus sequence of the expressed swine VH genes were compared with those of VH gene for humans (VHI, II, III), mouse (VHI, II, III), rabbit, and chicken, swine VH genes seem to have common ancestry with the human VHIII family, rabbit VH genes, and the single functional VH gene of the chicken. The leader peptides of all clones obtained by anchored PCR showed < 1% variability, and the deduced amino acid sequences from aa4 to aa25 in all 34 clones are identical. Framework (FR)1 and FR2 are conserved whereas FR3 shows greater variability. A total of 23 of 30 JH sequences were identical, suggesting preferential use of one JH. This frequent, putative JH sequence is not similar to any JH gene in humans. Identical genomic Southern hybridization patterns, each with 13 bands of differing intensity and regardless of stringency, were obtained when either a leader or a pan-specific VH probe was used. Analyses of individual genomic bands by single strand conformational polymorphism and sequence analysis suggest that the number of VHIII-related genes in the swine genome is < 20.