SummaryFifty-two rhinovirus strains were isolated from 672 cases of acute respiratory illness in children and adults and 4 strains were recovered from well controls. Respiratory disease cases among adults which yielded a rhinovirus were nearly all upper respiratory illness (common cold) whereas the cases among children often showed lower respiratory tract involvement. Nearly all cases revealed a homotypic neutralizing antibody response during convalescence. Among the total 56 strains isolated, 20 new serotypes were found. These 20 new types added to the 20 types described previously by our laboratory and 13 types recognized by other workers bring the total number of known serotypes to 53. The extreme diversity of serotypes among rhinoviruses goes far toward explaining the recurrent nature of the common cold and emphasizes the likely difficulty for development of effective vaccines for protection against this syndrome.We are indebted to Drs. S. M. Hoch, W. Freeborn, H. H. Palmer, Merck and Co., and to Drs....