The present invention relates to novel antibodies, particularly antibodies directed against deletion mutants of epidermal growth factor receptor and particularly to the type III deletion mutant, EGFRvIII. The invention also relates to human monoclonal antibodies directed against deletion mutants of epidermal growth factor receptor and particularly to EGFRvIII. Diagnostic and therapeutic formulations of such antibodies, and immunoconjugates thereof, are also provided.