Standardization of analytical methods in clinical chemistry is important because it facilitates interpretation of laboratory results. Many chemical determinations are now well standardized, but this is less common with immunoassays. Standardization requires availability of a primary standard and a reference method. Although standards are available for most clinically important analytes, reference methods for immunoassays are available only for haptens (1). The study of Haese et al. (2) in this issue aims at standardization of assays for human kallikrein 2 (hK2); it highlights many of the problems associated with standardization of sensitive immunoassays and with the use of recombinant proteins as standards. hK2 is a potentially important marker for prostate cancer, the most promising application being evaluation of tumor aggressiveness. Most prostate cancers grow slowly, with average tumor-doubling time being 2–4 years, but some tumors grow aggressively. Moreover, it is important to know whether a slightly increased prostate-specific antigen (PSA) value is caused by benign prostatic disease or by a cancer and, if so, whether this cancer needs to be cured. On the tissue level, PSA expression decreases with increasing tumor grade (and aggressiveness), whereas hK2 expression is constant or less affected (3). Several …