Immunoperoxidase methods permit the demonstration of a variety of antigens in routinely processed tissues. The morphological detail obtained by this procedure is comparable to orthodox haematoxylin and eosin stained sections, and so offers the diagnostic pathologist the opportunity of simultaneously identifying normal and neoplastic cells both by their morphological characteristics and by their antigenic constitution. The application of this method to a study of malignant lymphomas has contributed to current re-appraisal of classification criteria for this group of neoplasms. It is anticipated that, as specific antisera against a range of different tissue antigens become available, immunoperoxidase methods will have a similar impact upon the re-definition of morphologic criteria related to other neoplasms.