Abstract: A substantially pure and isolated novel form of prostate specific antigen (PSA) is provided. The novel form of PSA of the present invention comprises at least one clip at Lys 182 of the amino acid sequence of a mature form of PSA. Preferably, the novel form of PSA additionally comprises one or more clips at a location selected from a group consisting of Ile 1, Lys 145, and Lys 146. More preferably, the form of PSA contains at least two clips at Lys 145 and Lys 182 of the amino acid sequence of a mature form of PSA. The novel forms of PSA exist at an elevated level in patients suspected of having benign prostatic hyperplasia (BPH) and therefore may be used as a serum mark or an immunohistological marker to help distinguish BPH from prostate cancer. Antibodies recognizing the novel forms of PSA and immunoassays that detect and determine the novel forms of PSA of the present invention in a sample are also provided.