Standardization of determinations for prostate specific antigen (PSA) has become an important issue due to the widespread use of these determinations for prostate cancer screening. Standardization of this assay is complicated due to the occurrence of two major forms of PSA in serum, the free antigen and a complex between PSA and alpha 1-antichymotrypsin (PSA-ACT). These two forms of PSA are recognized differently by different antibodies, but by careful selection of antibodies, it is possible to design methods that measure each form equally. It is suggested, that standards for PSA and PSA-ACT should be prepared and established as international standards. Furthermore, reference methods should be established on the basis that these standards and carefully selected reference antibodies.