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The current issue of Applied Immunohistochemistry and Molecular Morphology (AIMM) includes the report of the Ad Hoc Committee on Immunohistochemistry Standardization.1 The Committee conducted much of its business remotely by telephone and by electronic communication, but was convened in person on 2 occasions, in Santa Barbara, CA in August 2006, and at Duck Key, FL in January 2007, under the sponsorship of Dako Corporation in the first instance, and the First Annual Course on Immunohistochemistry and Molecular Morphology in the second. The Ad Hoc Committee consisted of a number of “volunteers” from diverse backgrounds, who previously had held informal discussions at different pathology meetings, and who shared a common interest in improving the overall quality of immunohistochemical methods. The report includes 14 recommendations, grouped into the 3 areas of Preanalytic, Analytic, and Postanalytic issues, and provides a comprehensive bibliography for those interested in the field. The report is presented for rapid publication in AIMM in keeping with the “raison d'etre” of the Journal, namely its focus upon the practical application of immunohistochemical and related methods in diagnostic pathology.Over several decades immunohistochemistry has evolved from a methodologic curiosity, of occasional research interest, to a technique that is in widespread use in surgical pathology, and is considered to be essential in many areas of cancer diagnosis and classification. Today, there is a resurgent interest in assuring the reproducibility of the method, even to the point of upgrading it from a “stain” to a tissue-based “immunologic assay.” If accomplished, this change would make possible true quantification of analytes in tissue sections, analogous to the use of the enyzyme-linked immunosorbent assay method in the clinical laboratory, which employs essentially the same reagents and similar principles, but is subject to much more rigorous control at all levels.2,3Immunohistochemistry gives a tinctorial reaction that is readily viewed by routine light microscopy, leading pathologists to categorize the result as nothing more than a novel “special stain,” akin to a trichrome stain or a periodic acid-Schiff stain. The introduction of the hybridoma method4 yielded a bounty of new antibodies, dozens of new “stains,” a burgeoning crop of new investigators, innovative variants of the method, new commercial vendors, easy to use “staining kits,” and even “automated stainers.” Over the last 2 decades the growth of literature in the field was explosive; it was an exciting time. One unintended consequence was that immunohistochemical stains were performed with beguiling ease in growing number of laboratories, with minimal attention to specimen acquisition, sample preparation (fixation), protocol, and controls, following a “modus operandi” that for more than a century had sufficed in the histopathology laboratory for an hematoxylin and eosin stain. As a result reproducibility suffered.From the very beginning of immunoperoxidase-based studies, describing the immunohistochemical demonstration and distribution of various “antigens” in formalin-fixed tissues, findings were quite readily reproduced by other investigators; to be precise, they were reproduced, but they were not strictly reproducible. Thus, a tinctorial reaction (stain) might be reproduced by different investigators, but the intensity, distribution, and overall quality were inconsistent, from laboratory to laboratory, from day to day, from tissue to tissue within the same laboratory, and even in different regions of a single tissue section. This observed variability was attributed to uncertain quality of the primary antibody (from the same or different sources), to vagaries of technique, the aptitude or ineptitude of the investigator, or to differences in fixation, or lack thereof.A number of workshops were convened over the years to examine these issues.