Enzymes, either acid phosphatase or horseradish peroxidase, were conjugated to antibodies with bifunctional reagents. The conjugates, enzymatically and immunologically active, were employed in the immunohistochemical localization of tissue antigens utilizing the reaction product of the enzymatic reaction as the marker. Tissues reacted with acid phosphatase-labeled antibodies directed against basement membrane were stained for the enzyme with Gomori's method, and those reacted with peroxidase-labeled antibody were stained with Karnovsky's method. The reaction products of the enzymes localized in the basement membrane. Unlike the preparations of the fluorescent antibody technique, enzyme-labeled antibody preparations were permanent, could be observed with an ordinary microscope, and could be examined with the electron microscope. In the latter, specific localization of antibody occurred in the basement membrane and in the endoplasmic reticulum of cells known to synthesize basement membrane antigens. The method is sensitive because of the amplifying effect of the enzymatic activity. The ultrastructural preservation and localization were better with acid phosphatase-labeled antibody than with peroxidase-labeled antibody, but acid phosphatase conjugated antibody was unstable and difficult to prepare. Peroxidase-antibody conjugates were stable and could be stored for several months at 4 degrees C, or indefiniely in a frozen state.