hi Marek's disease virus (MDV) is the causative agent for Marek's disease in chickens. The virus can be grown in susceptible cells in the laboratory, producing small discrete lesions. These lesions or plaques can be viewed and counted with the aid of a microscope, thereby determining how many infectious viruses there may be in a sample or vaccine vial. The difficulty is that the plaques are very small and are often indistinguishable from normal cells. We solved this problem by utilizing some chemicals and an antibody specific for MDV to develop a staining technique that stains MDV plaques. The procedure is simple and fast, resulting in plaques appearing as black spots that can be easily seen and counted with the naked eye. In addition, counting black plaques appears to be more accurate than the counting of conventional unstained plaques