The present invention relates to the production of a transgenic bovine which comprises a genetic modification that results in inactivation and loss of expression of its endogenous antibodies, and the expression of xenogenous antibodies, preferably human antibodies. This is effected by inactivation of the IgM heavy chain expression and, optionally, by inactivation of the Ig light chain expression, and by the further introduction of an artificial chromosome which results in the expression of non-bovine antibodies, preferably human antibodies.