The present invention relates to monoclonal antibodies that bind to the extracellular domain of prostate-specific membrane antigen (PSMA), hybridoma cell lines producing the antibodies, and methods of using such antibodies for diagnosis and treatment of cancer. In particular, thirty-five monoclonal antibodies reactive with PSMA expressed on the cell surface are exemplified. Additionally, the present invention relates to a novel protein variant (PSM') of PSMA detected by a number of the antibodies of the invention. The hydrolase activity of PSMA and PSM' allows the use of an immunoenzymatic assay for their detection.