Naturally occurring antibody repertoires of cattle (Bos taurus) include a group of IgMlambda antibodies with exceptionally long complementarity-determining region 3 of the heavy chain (CDR3H) segments, containing multiple Cys residues. These massive CDR3H segments will greatly influence the tertiary and quaternary structures of the bovine IgM combining sites. As an antibody’s combining site is formed by both heavy and light chains, we have analyzed the nucleotide sequences and structural properties of the lambda-light chains that pair with micro -heavy chains containing exceptionally long CDR3H. There appears to be an exquisite selective pressure for the use of three V(lambda)1 genes (V(lambda)1x and two new V(lambda)1d and V(lambda)1e genes) in IgM with unusually long CDR3H. The V(lambda)1d and V(lambda)1e genes are similar to each other, but diverge from the other V(lambda)1 genes into two closely related subfamilies. The available bovine V(lambda) genes are classified into three V(lambda) gene families: V(lambda)1, V(lambda)2 and V(lambda)3 based on nucleotide similarity >/=80%. Further, analysis of total Ser content and positions of Ser residues in the sequences was found to be sufficient to classify the cattle V(lambda)1 subfamilies. Patterns of Ser residues differ for V(lambda) domains from ruminant species (e.g. cattle, sheep and goats) and other mammals (e.g. humans and mice). These ‘Ser signatures’ can be used to track divergent evolution in lambda-light chains. Interestingly, Ser90L in complementarity-determining region 3 of the light chain (CDR3L) occurred in all V(lambda) domains that pair with V(H) regions containing exceptionally long CDR3H. A structural role for Ser90L was revealed in homology models of V(lambda) domains, i.e. to hold the ascending polypeptide of CDR3L in a relatively tight space between the N-terminal segment and residues from CDR1L. The CDR3L of V(lambda) domains also occupied smaller volumes if paired to V(H) domains with extremely long CDR3H (>/=48 residues), and were more variable in their conformation and filled larger volumes if CDR3Hs were </=22 residues. Thus, the role of the lambda-light chains in these unusual cattle antibodies is probably to act as a relatively featureless supporting platform for the extremely long CDR3H regions, which undoubtedly are dominantly involved in binding to an antigen.