The wide variances between prostate specific antigen (PSA) values for various assays and the demonstration that many of these differences are due to calibration differences has resulted in efforts to develop standards for PSA. Two major efforts are underway in the USA. The National Committee for Clinical Laboratory Standards (NCCLS) has published proposed guidelines for the purification and characterization of PSA and PSA-ACT (alpha-1-antichymotrypsin) complexes for primary standards. Furthermore, the Second Stanford PSA Conference proposed a mixture of 90% PSA-ACT and 10% PSA (90:10 standard) with a biochemically defined concentration to calibrate total PSA assays. Significant improvements in agreement between assays was observed with the 90:10 standard as compared to results with kit calibrators. The NCCLS has reviewed and adopted the 90:10 proposal. Thus, biochemically defined standards are preferred over immunoassay defined standards due to differences in assays and laboratory methods. The use of the 90:10 standard will be a major step towards improving the agreement between PSA immunoassays.