Plaque assay is a quantitative assay that determines the number of infective virions in the viral stock or the infected cells. Plaques are essentially virus-infected cells that produce progeny virus and infect adjacent cells. The cells producing progeny virus die and detach from the dish, leaving an empty space in the confluent monolayer of cells. Unlike other viruses, rhinovirus does not form characteristic round plaques but shows small clear areas of different shapes surrounded by dying cells. Rhinoviruses form plaques only in highly susceptible H1HeLa cells but not in their primary target, airway epithelial cells. The plaque assay to determine the number of infective virions works only for rhinovirus species A and B, but not C, because the latter does not infect H1HeLa cells. Here we describe a method to quantify the infective virions by plaque assay for rhinovirus speciesA and B.