Vaccines are the primary means by which to prevent, control, or eradicate infectious diseases. While many vaccines have been successfully developed and have resulted in enormous medical and veterinary benefit, there are certain viruses that have eluded effective vaccine development. Generally, viruses with multiple strains or that have high mutation rates can evade neutralizing antibodies, as their surface determinants are variable and result in the inability of neutralizing antibodies raised against one strain to bind and neutralize alternative strains. Certain rare epitopes, however, are required for viral infection and are conserved across strains. Interestingly, neutralizing antibodies against these rare epitopes tend to have long CDR H3 regions. In the case of HIV, long CDR H3s can pierce the viral glycan shield and reach into the conserved epitope on the gp120 spike protein. While long CDR H3 regions in human antibodies are infrequent in the repertoire, cattle routinely produce long (20-40 amino acids) and ultralong (40-70 amino acids) CDR H3 regions that have unique “stalk” and “knob” structural features that protrude far from the antibody surface. Therefore, cattle may be an excellent model organism to identify and define new and conserved neutralizing epitopes in these challenging viruses. Indeed, in preliminary experiments we have found that cattle make a robust and broadly neutralizing antibody response to the HIV gp120 antigen. Here we propose to use the unique cow antibody repertoire to define new conserved neutralizing epitopes on two viruses of great importance to human and animal health, HIV and BVDV. Effective vaccines against both of these viruses have been a major challenge to develop. We will immunize animals against these viruses, generate monoclonal antibodies that neutralize the virus as well as related strains, and molecularly map the antigen-antibody interaction using mutagenesis and structural biology techniques. Definition of new conserved epitopes could lead to engineered epitope-specific vaccines. Thus, the outcomes of this proposal could enable generation of next-generation vaccines for these two viruses, but could also have broad utility in vaccine development against other challenging viruses in the future.