Hybridomally produced monoclonal antibodies specifically immunoreactive with the glycoprotein carrying the CA 125 epitope. Monoclonal antibodies recognize both high and low molecular weight subunits of the antigen, and identify the antigen in the cytoplasm and the extracellular matrix of CA 125 producing cells. An immunoassay for the detection of CA 125 utilizing the monoclonal antibodies is described.