Cryostat sections were incubated with two different monoclonal antibodies directed either against two different cells or against the same cell. For visualization of the first antibody, the alkaline phosphatase-antialkaline phosphatase (APAAP) technique was used, which rendered a bright red reaction product. For visualization of the second monoclonal antibody, the immunogold-silver-staining (IGSS) technique was used, which resulted in a black reaction product. The combination of an enzyme technique with a technique involving metals (gold and silver) seemed ideal since the reaction product of either technique could be detected easily.