A transgenic chicken is disclosed having disrupted endogenous immunoglobulin production. In one embodiment, a targeting construct is stably integrated into the genome of the chicken by homologous recombination in embryonic stem cells, and injection of the engineered embryonic stem cells into recipient embryos, thereby knocking out the endogenous immunoglobulin gene locus in resulting animals. The targeted disruption of the locus in embryonic stem cells is particularly useful in combination with the insertion of genetic elements encoding exogenous immunoglobulin molecules. After these chickens are cross-bred, a line of chickens is produced that has a reduction of endogenous immunoglobulin molecule production.