To define the number of rhinovirus serotypes, cross neutralization tests and characterization studies were completed on 25 candidate prototype rhinoviruses submitted to a third phase of a collaborative program. Based on the results, 11 distinct prototype strains were designated and the numbering system was extended to include 100 rhinoviruses. In addition, recent evidence indicates that over 90% of rhinoviruses isolated in three areas of the country could be typed with antisera for rhinovirus types 1-89.