A truncated epidermal growth factor receptor (EGFR) expressed from a rearranged and amplified EGFR gene is present at high frequency in gliomas. In this work we show that when this receptor is expressed in NIH3T3 fibroblasts it is partially activated and confers tumorigenicity to this cell line in vivo but no growth advantage in in vitro anchorage-independent growth assays. Because the mutation occurs in the extracellular domain of the receptor, it can be considered to represent a glioma-specific tumour marker. Here we demonstrate that 2 monoclonal antibodies, DH1.1 and DH8.3, raised to a synthetic peptide spanning the unique junctional sequence, can recognise the mutant receptor but not the normal receptor in both denatured and native states. Furthermore, radiolabelled antibody DH8.3 successfully targets tumours expressing this antigen in nude mice.