The rat neu proto-oncogene, which is a putative growth factor receptor closely related to the epidermal growth factor receptor, can be activated in vivo by a single point mutation in the sequence encoding its transmembrane region. The human homologue of neu, c-erbB-2, can be activated in vitro to an oncogenic form by point mutations in the same relative position in the gene. We have sought the presence of such activating mutations in a series of 100 cases of human breast cancer by polymerase chain reaction amplification and sequence-specific oligonucleotide hybridization, and also by a designed restriction fragment length polymorphism strategy in cases with Southern blot evidence of c-erbB-2 amplification. No evidence of activating point mutations in the c-erbB-2 protooncogene was found in any of these cases.