The rhinovirus group comprises at least 115 accepted serotypes and more that are not yet classified. These picornaviruses are small (20 to 30 nm), naked particles containing sin-gle-stranded, positive-sense RNA. They are distinguished from enteroviruses by their acid lability and an optimum temperature of 33 C for in vitro replication. This temperature approximates that of the nasopharynx in the human host and may be a factor in the local-ization of pathologic findings at that site. Rhinoviruses are most consistently isolated in cultures of human diploid fibroblasts. The receptor for most rhinoviruses (and some cox-sackieviruses) is glycoprotein intercellular adhesion molecule 1 (ICAM-1), a member of the immunoglobulin supergene family. ICAM-1 is best known for its role in immunologic cell adhesion; its ligand is lymphocyte function-associated antigen-1.