Immunohistochemical techniques have become commonplace adjunctive aids in anatomic pathology. Although much has been written describing modifications of the basic techniques, sensitivity, and specificity of reagents, little has been published regarding the interlaboratory variability in immunostain results on a given test sample. The Cell Markers Survey of the College of American Pathologists was organized to address this question of interlaboratory variability and to disseminate information on the techniques and reagents currently available.