The advent of immunoperoxidase histochemistry techniques has allowed the more convenient visualization by light microscopy of antigenic cellular constituents. Application of the unlabeled and labeled antibody enzyme methods to the localization of carcinoembryonic antigen in conventionally prepared tissue specimens has revealed the feasibility of using immunoperoxidase techniques in routine diagnostic pathology. The methods involved are well defined and once antibody specificity and antigen stability are determined, they are adaptable to the identification of other immunological markers that may appear in malignant tissue specimens.