Standardization of immunoassays for estradiol-17β (E2) for accuracy was considered in the “Estradiol International Workshops” (1). There, industry asked our laboratory to provide them with a means to evaluate the accuracy/specificity of their E2 test systems. In answer to this demand, we developed and made commercially available an accuracy-based panel of patient specimens. The results of the method comparisons subsequently performed by industry are presented below. Perspectives to use this exercise as a basis for further international standardization efforts are given.