A method of preparing a pharmaceutical composition comprising an antibody and a diluent, excipient or carrier, the method comprising (a) immunising a fertile rodent which is a mouse or rat with a human target antigen such that the mouse or rat produces antibodies; and (b) isolating from the mouse or rat an antibody that specifically binds to said antigen and/or isolating a nucleotide sequence encoding at least the heavy and/or the light chain variable regions of such an antibody; and optionally subsequently joining the variable regions of said antibody to a human constant region, wherein the fertile mouse or rat is optionally male, and is homozygous for a transgenic antibody heavy chain locus, the mouse or rat having a genome that (a) comprises each heavy chain locus on a respective copy of a first chromosome; and (b) is inactivated for endogenous antibody heavy chain expression; wherein each first chromosome of the genome comprises (c) a transgenic antibody heavy chain locus comprising one or more human VH gene segments, one or more human D gene segments and one or more human JH gene segments operably connected upstream of a mouse or rat heavy chain constant region, optionally Cmu and/or Cgamma; (d) a deletion of all or part of the endogenous heavy chain VDJ region of said chromosome to inactivate endogenous antibody heavy chain expression, wherein the deletion includes ADAM6; wherein the genome comprises (e) an insertion of one or more expressible ADAM6 genes; wherein the genome comprises inserted expressible mouse ADAM6a and ADAM6b genes; or wherein the genome comprises an inserted expressible rat ADAM6 gene, wherein each ADAM6 nucleic acid sequence is inserted so that it is operably connected to a promoter for expression, and wherein the composition is contained in an intravenous bag or a container connected to an IV syringe. (a) immunising a fertile rodent which is a mouse or rat with a human target antigen such that the mouse or rat produces antibodies; and (b) isolating from the mouse or rat an antibody that specifically binds to said antigen and/or isolating a nucleotide sequence encoding at least the heavy and/or the light chain variable regions of such an antibody; and optionally subsequently joining the variable regions of said antibody to a human constant region, wherein the fertile mouse or rat is optionally male, and is homozygous for a transgenic antibody heavy chain locus, the mouse or rat having a genome that (a) comprises each heavy chain locus on a respective copy of a first chromosome; and (b) is inactivated for endogenous antibody heavy chain expression; wherein each first chromosome of the genome comprises (c) a transgenic antibody heavy chain locus comprising one or more human VH gene segments, one or more human D gene segments and one or more human JH gene segments operably connected upstream of a mouse or rat heavy chain constant region, optionally Cmu and/or Cgamma; (d) a deletion of all or part of the endogenous heavy chain VDJ region of said chromosome to inactivate endogenous antibody heavy chain expression, wherein the deletion includes ADAM6; wherein the genome comprises (e) an insertion of one or more expressible ADAM6 genes; (a) comprises each heavy chain locus on a respective copy of a first chromosome; and (b) is inactivated for endogenous antibody heavy chain expression; wherein each first chromosome of the genome comprises (c) a transgenic antibody heavy chain locus comprising one or more human VH gene segments, one or more human D gene segments and one or more human JH gene segments operably connected upstream of a mouse or rat heavy chain constant region, optionally Cmu and/or Cgamma; (d) a deletion of all or part of the endogenous heavy chain VDJ region of said chromosome to inactivate endogenous antibody heavy chain expression, wherein the deletion includes ADAM6; wherein the genome comprises (e) an insertion of one or more expressible ADAM6 genes;