All immunocytochemical techniques are based on the same principle of incubating the target antigen with an appropriate antibody solution, but they may incorporate one from a range of different microscopically dense markers for visualizing, the sites of binding. Light microscope immunocytochemistry was initiated by the classical work of Coons and coworkers in 1950, who developed the immunofluorescence technique for antigen localization (1 ,2 ). This was followed some time later in 1966 with the introduction by Nakane and Pierce of the lmmunoperoxidase procedure (3 ), after which there have been various attempts to increase the sensitivity of techniques for localizing tissue antigens, including peroxidase antiperoxidase (PAP) (4 ), avidin-biotin complex (5 ), and alkaline phosphatase antialkaline phosphatase (APAAP) (6 ). These techniques are all still routinely used for immunocytochemistry.