Accurate determination of prostate specific antigen is important because of the increasing use of prostate specific antigen for diagnosis, follow-up and screening of prostate cancer. Standardization of this assay is complicated by the occurrence in serum of two major molecular forms of prostate specific antigen, free prostate specific antigen and a complex between prostate specific antigen and alpha 1-antichymotrypsin. These two forms of prostate specific antigen are recognized differently by many but not all antibodies. Thus, it is possible and desirable to develop methods that measure each form equally. To achieve better comparability, it is also necessary to prepare international standards for prostate specific antigen and its complex with alpha 1-antichymotrypsin. Furthermore, reference methods should be established which use these standards and carefully selected reference antibodies.