Previous studies have assigned 88 human rhinovirus (HRV) serotypes to major and minor receptor groups. Extension of these studies to include the remaining 14 unassigned serotypes indicated that 13 serotypes belong to the major group since their infection of HeLa cells is completely blocked by a monoclonal antibody that recognizes the major group receptor. This result indicates that the major group now accounts for 91 of the 102 known serotypes, while the minor group contains 10 serotypes. One serotype, HRV-87, appears to utilize neither the major nor minor group receptor and may represent a third receptor group. HRV-87 attachment cannot be blocked by other serotypes and displays a binding tropism similar to but distinct from minor group viruses. Unlike major and minor group serotypes, HRV-87 attachment and infection requires the presence of sialic acid on cellular receptors.