We analyzed VDJ and VJ rearrangements in IgM-secreting B lymphocytes from a cow infected with bovine leukemia virus (BLV). BLV causes expansion of CD5(+) and IgM(+) B lymphocytes regardless of antigen specificity. The data showed that single point mutations contribute to the diversification of IgM antibodies. The most striking observation, however, is that approximately 9% of theVDJ rearrangement in IgM-secreting B cells encode an exceptionally long third complementarity-determining region of the heavy chain (CDR3H; 56 to 61 amino acids) with multiple cysteine residues. Such an exceptionally long CDR3H is the first ever to be documented for an antibody in a species. These VDJ rearrangements encode functional IgM antibodies as some of these show polyspecific reactivity. The presence of even-numbered cysteine residues in the CDR3H may provide hitherto unknown configurational ability to the antigen combining site via intra-CDR3H disulfide bridging. In addition, the VDJ rearrangements encoding exceptionally long CDR3H paired with either novel V(lambda)1 or V(x)1x genes, earlier noted not to be expressed. Overall, these experiments provide evidence that somatic hypermutations and generation of an exceptionally long CDR3H contribute to the diversification of IgM antibodies in cattle.