We have developed TransChromo (TC) technology, which enables the introduction of megabase-sized segments of DNA into cells. We have used this approach to derive mice that carry megabases of human DNA by the use of a human chromosome fragment (HCF) as a vector. TC technology has been applied to the construction of the TC Mouse,trade mark which incorporates entire human immunoglobulin (hIg) loci. TC Mouse expresses a fully diverse repertoire of hIgs, including all the subclasses of IgGs (IgG1-G4). Immunization of the TC Mouse with various human antigens produced antibody responses comprised of human antibodies. Furthermore, it was possible to obtain hybridoma clones expressing fully human antibodies specific for the target human antigen. However, because of the instability of the Igkappa locus-bearing HCF2, the efficiency of hybridoma production was less than one-tenth of that observed in normal mice. An instant solution to this problem was to cross-breed the Kirin TC Mouse carrying the HCF14, which was stable in mouse cells, with the Medarex YAC-transgenic mouse carrying about 50% of the hIgVkappa gene segments as a region that is stably integrated into the mouse genome. The resulting mouse, dubbed the KM Mouse, performed as well as normal mice with regard to immune responsiveness and efficiency of hybridoma production. Another application of TC technology is the production of polyclonal antibodies in large animals such as chickens and cows. To test the efficacy of human polyclonal antibodies derived from TC animals, feasibility studies were performed using antisera and purified gamma-globulin from TC mice immunized with Pseudomonas aeruginosa, methicillin-resistant Staphylococcus aureus (MRSA), or Japanese encephalitis virus (JEV). The TC mouse-derived antisera and gamma-globulin showed a much higher titer and efficacy in terms of the neutralizing activity of the pathogens in vitro and in vivo than either human serum or gamma-globulin prepared from human blood.