A hemolytic plaque assay is a technique used to show that certain antibodies are secreted by antibody-making cells when these cells are mixed with red blood cells. The mixture is spread onto a plate of agar; if a plaque appears, it means that the red blood cells have been destroyed (by hemolysis) by the antibodies.

From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.