Human antibodies are produced in transgenic chickens having functional genetic components of the human immune system stably integrated in the genome. Techniques are described to integrate unrearranged human heavy and light chain immunoglobulin loci into the genome of the transgenic chicken. The functional description of the endogenous immunoglobulin loci yields an animal whose antibody repertoire in response to antigen is fully human. In the preferred embodiment, the human immunoglobulin locus is at least as large as the native human locus and exhibits functional class switching to yield IgG isotype antibodies. In addition to monoclonal antibodies secreted from immortalized B cell populations, polyclonal antibodies may be obtained from eggs laid by the transgenic chickens of the invention.