Towards the goal of producing fully human polyclonal antibodies (hpAbs or hIgGs) in transchromosomic (Tc) cattle, we previously reported that Tc cattle carrying a human artificial chromosome (HAC) comprising the entire unrearranged human immunoglobulin (Ig) heavy-chain (h IGH), kappa-chain (h IGK), and lambda-chain (h IGL) germline loci produced physiological levels of hIgGs when both of the bovine immunoglobulin mu heavy-chains, b IGHM and b IGHML1, were homozygously inactivated (b IGHM −/− , b IGHML1 −/− ; double knockouts or DKO). However, because endogenous bovine immunoglobulin light chain loci are still intact, the light chains are produced both from the h IGK and h IGL genomic loci on the HAC and from the endogenous bovine kappa-chain (b IGK) and lambda-chain (b IGL) genomic loci, resulting in the production of fully hIgGs (both Ig heavy-chains and light-chains are of human origin: hIgG/hIgκ or hIgG/hIgλ) and chimeric hIgGs (Ig heavy-chains are of human origin while the Ig light-chains are of bovine origin: hIgG/bIgκ or hIgG/bIgλ). To improve fully hIgG production in Tc cattle, we here report the deletion of the entire b IGL joining (J) and constant (C) gene cluster (b IGLJ1-IGLC1 to b IGLJ5-IGLC5) by employing Cre/loxP mediated site-specific chromosome recombination and the production of triple knockout (b IGHM −/− , b IGHML1 −/− and b IGL −/− ; TKO) Tc cattle. We further demonstrate that b IGL cluster deletion greatly improves fully hIgGs production in the sera of TKO Tc cattle, with 51.3% fully hIgGs (hIgG/hIgκ plus hIgG/hIgλ).