In order to study the requirements for a human antibody repertoire to be engineered in transgenic animals, we have created mouse strains that express human immunoglobulin genes from 100 kb of cosmid DNA. Undisrupted integration of the human DNA in the mouse germ line, encoding germ-line V,D,J and the Cmu constant region, was achieved, and proved sufficient for the production of human IgM in the mouse serum. Co-integration of one cosmid (containing the VH6 gene and a 36-kb region of the J-proximal D cluster) with a second cosmid (containing VH26, a further 38-kb of the D cluster, JH and Cmu) results in V-D-J rearrangements in the transgenic mice, similar to those seen in human DNA, were found only in spleen but not in thymus. Random hybridomas made from these transgenic mice show heterogeneous rearrangements of the human transgenes. Sequences of V-D-J units derived from transgene rearrangements reveal extensive N-region and apparent D segment diversity. These results show that utilisation of human Ig genomic segments does occur in transgenic mice, paving the way for the derivation of a mouse strain that makes authentic human antibodies from inserted heavy and light chain gene loci.