Rhinoviruses, prominent agents of the common cold syndrome in man, are small ribonucleic acid (rna) viruses resembling enteroviruses in their physicochemical properties except for high density and lability to acid pH. Rhinoviruses are propagated in human and monkey cells. Highest titers of virus are obtained in HeLa cell cultures. Rhinoviruses produce characteristic cytopathic effect in diploid fibroblasts. A plaque assay in HeLa cells is useful for their titration. The rhinovirus group includes many serotypes. Although rhinoviruses cause predominantly upper respiratory tract symptoms, they may on occasion infect the lower respiratory tract. Volunteers with specific antibody, when challenged with homotypic rhinovirus, are protected against the common cold.