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Chromogenic detection methods often use a conjugated enzyme to visualize epitope-antibody interactions. When using this method of detection, the endogenous activity of the same enzyme must be blocked. For example, protocols that include horseradish peroxidase (HRP) or alkaline phosphatase (AP) may require reagents to prevent non-specific signals. Tissues such as kidney, liver, or vascular areas with red blood cells, contain endogenous peroxidase activity. Peroxidase blocking reagents formulated with 3-10% H2O2 can be used to prevent endogenous peroxidase from cleaving the substrate. Endogenous AP found in intestine, kidney, lymphoid and other tissue can be blocked with 1 mM Levamisole. The intestinal form of AP is unaffected by Levamisole but can be blocked by using 1% acetic acid.