Immunohistochemical studies employ an unla-beled antibody to a specific tissue antigen fol-lowed by treatment in one or more steps with an enzyme-labeled antibody. These studies are extremely versatile and can be used to detect an ever-expanding number of antigens. Depending on the specific antigens, they can be performed on fresh-frozen or formalin-fixed, paraffin-embedded tissues. The effectiveness of immuno-histochemistry depends on the integrity, stability, and availability of the target antigen. Three steps can improve the results of your immunohisto-chemical staining. First, sample a viable and rep-resentative area of the process to be studied. Immunohistochemical stains of necrotic material are practically worthless. Second, choose the ap-propriate method of stabilizing the tissue. While formalin is generally a good fixative for most im-munohistochemical stains, some stains work best in other fixatives, and other stains work only on fresh-frozen tissue. Finally, do not overfix the tissue. Overfixation may result in the loss of antigenicity. In general, tissue fixation for longer than 24 hours compromises immunohisto-chemical analysis.