Immunoperoxidase methods are now used extensively in surgical pathology. The basic technique serves both as an investigative tool and as a type of special stain for immunohistochemical studies, and often yields a definitive diagnosis when orthodox histochemical methods are of no avail. Many variations of the technique have been developed since the basic immunoperoxidase method was first applied to "routinely processed" tissues in 1974. Use of each should take into account its particular strengths and weaknesses.