Conjugated and unlabeled peroxidase antibody methods have proven to be quite satisfactory in localizing sites of antigen-antibody reaction. The use of avidin-biotin-peroxidase complex (ABC), as well as protein A, can contribute significantly to the field of immunohistochemistry. The sensitivity and specificity of several immunohistochemical methods is compared. In general, the ABC method produced the most intense staining and the least background staining of any method tested. The unlabeled antibody (peroxidase-antiperoxidase: PAP) method also yielded satisfactory results, but it was less intense than the ABC method. In comparison to the PAP method, the indirect conjugated method presented slightly inferior staining intensities and significantly higher background staining. Protein A techniques produced a range of staining sensitivities similar to or inferior to the PAP technique. The main disadvantage in using protein A is that it reacts with intrinsic immunoglobulin (Ig) G, thus producing an intense background. Therefore, its use is not recommended on tissues that have either abundant immunoglobulins in their interstitium or numerous IgG-containing plasma cells.