Validation of a clinical laboratory test means confirmation, through a defined process, that the test performs as intended or claimed. Proper validation provides reasonable, but not absolute, assurance that a test is performing as anticipated. There is no single, universally acceptable procedure for validating clinical laboratory tests. The design of a validation protocol requires professional judgment, and validation schemes must take into account the test's intended use, other claims made about the test, and risks that may prevent the test from meeting performance claims. This article provides guidance on analytic (technical) validation procedures that we believe should be used by laboratories offering estrogen receptor (ER) and progesterone receptor (PgR) assays by immunohistochemical (IHC) methods. We describe minimal procedures for initially validating the tests before they are placed in clinical service. We also discuss labeling requirements (language) applicable to reporting and to claims a laboratory may choose to make about its assays. A separate guideline (1) describes required elements of an ongoing quality management program for hormone-receptor testing by IHC methods, including daily quality control testing, external proficiency testing, and general controls applied to laboratory personnel, equipment, reagents, and other aspects of laboratory service.