Fast Red chromogenic substrate is an alkaline phosphatase substrate used for immunohistochemical (IHC) revelation. When reacted with alkaline phosphatase, Fast Red produces an insoluble red precipitate at the reaction site on tissues and cells.

AP hydrolyzes the naphthol phosphate esters to phenolics and phosphates. The phenols couple to colorless diazonium salts to produce an insoluble colored azo dye.

The appearance of the staining can be followed by observation under the microscope.

The precipitate formed by Fast Red in the presence of alkaline phosphatase is soluble in alcohol.

Fast Red can be used to make multiple labeling with other chromogens.