The present invention relates to methods for increasing the diversity of monoclonal antibodies produced against an antigen. The methods of the invention utilize immunization of a murine host defective in one or more enzymes involved in a post-translational modification of a polypeptide or a modification of a lipid, wherein said modification is exposed on a cell surface. The invention also relates to monoclonal antibodies produced by these methods and which are not produced when a normal mouse is immunized with the same antigen. The invention further relates to compositions comprising these monoclonal antibodies, as well as to such monoclonal antibodies bound or conjugated to a toxin, a detectable marker or to a solid support.