Somatic cell hybrids were made from mouse myeloma cells and spleen cells derived from BALB/c mice immunized with homogenized epithelial fractions of BPH. The screening by immunoperoxidase staining on human prostate and non-prostate tissue resulted in one monoclonal antibody identifying a prostate specific antigen. Upon SDS-PAGE and Western blot this antigen exhibited a single band at the position of 34 kDa molecular weight. The immunoreactivity of the prostate antigen was found to be localized exclusively in the epithelial lining of ducts and secretions of normal prostate, BPH and prostate cancer. Anti-p34 antibody reacted with an antigenic determinant on the PSA molecule cancer. Anti-p34 antibody reacted with an antigenic determinant on the PSA molecule and inhibited the binding of Anti-PSA antibody to PSA by about 80 to 90% in the RIA.