Antibodies are encoded by a limited number of germline gene segments that undergo somatic diversification through rearrangement and mutation. Because these mutation processes are efficient, it is widely believed that there is little environmental selection pressure for the maintenance of specific antibody gene sequences. We have performed pairwise comparisons of known germline (as opposed to somatically generated) antibody VHelements with the hope of identifying conserved structural features common to sets of VHgene segments. These studies reveal that VHfamilies arose prior to the mammalian radiation and have since been conserved, that this conservation appears to reflect selection at the level of protein sequence, and that the conserved regions are discretely localized on a solvent-exposed face of the heavy chain, at some distance from the antibody combining site. A family-specific region was also identified within the recombinase recognition sequences. Our results provide a context for theories that address the physiological significance of variations in VHfamily utilization during the development of the immune repertoire. © 1990 Oxford University Press.