A series of monoclonal antibodies specific for the extracellular domain of the human neu gene product (p185) have been produced. The generation of these monoclonal antibodies, and their biochemical and immunological characterization is described. The immunization protocol utilized a series of injections of NIH3T3 cells, cyclophosphamide, and a neu transfected NIH3T3 cell line (designated 18-3-7) which expressed the full length human neu-encoded protein. This immunization regimen induced an immune response to the extracellular portion of p185 on the 18-3-7 cells. A panel of ten hybridomas were identified which secreted monoclonal antibodies with a variety of epitope specificities, and reacted with p185 in a number of different experimental formats. As the neu gene product has been associated with human breast cancers, a series of monoclonal antibodies such as these could prove useful in the diagnosis, prognosis and/or treatment of these human malignancies.