A transgenic organism is provided comprising a polynucleotide construct encoding an intracellular antibody which disrupts the catalysis of the production of the xenoantigen galactose α 1,3 galactose and/or a polynucleotide construct which encodes an intracellular antibody which binds specifically to a retrovirus protein, such as a PERV particle protein. Also described are methods for the production of such organisms. Cells, tissues and organs of the transgenic organism may be used in xenotransplantation.