Abstract
A nonspecific staining of mast cells by avidin-biotinylated peroxidase complexes (ABC) has been observed and related to an ionic binding of the basic residues of avidin (isoelectric point at pH 10) and peroxidase to the sulphate groups of heparin. This affects the correct interpretation of the results of the immuno-peroxidase ABC procedure, especially in mast cells-rich areas such as the lymphoid tissues. The spurious staining can be prevented by using the ABC solution at pH 9.4 (instead of pH 7.6 as usually recommended): this high pH does not affect the previous binding of primary antibodies nor the affinity of avidin to biotin. The modified ABC procedure provides a clear background and a sharp specific staining and can be recommended for routine use.