Abstract: The present invention provides a novel complex of hK2 and PI-6 and methods of using the novel complex and its constituents. The novel complexes of hK2 and PI-6 of the present invention and the PI-6 exist at an elevated level in prostate cancer tissues. PI-6 also exists at an elevated level in other types of cancer cells. Therefore, the hK2-PI6 complexes and PI-6 of the present invention may be used as a serum marker for detecting cancers, particularly prostate cancer. They may also be used as an immunohistological marker to detect prostate cancer tissues. In accordance with the present invention, the hK2-PI6 complexes of the present invention may be detected in patient tissue samples by immunohistochemical and/or in patient fluid samples by in vitro immunoassay procedures. Diagnostic methods for detecting the existence of prostate cancer is also provided.