Chimeric transgenic immunoglobulin (Ig) mice are provided that comprise an altered heavy chain locus in which endogenous mouse D and J segments have been deleted and a human heavy chain Ig transgene is inserted at that location such that the transgenic Ig mouse expresses an antibody repertoire that utilizes human VH and mouse VH in heavy chains, each linked to human DH and JH segments, leading to enhanced diversity. Methods of preparing and using the transgenic animals (e.g., to raise antibodies) are also provided.