The frequency of the common cold has been though to be due to the existence of more than 89 different serologic types of rhinoviruses. However, in the civilian and military population studied in this area from 1962 through 1970, types 1A, 1B, 2, 23, 29, 30 and 31 accounted for 81 percent of 487 rhinoviruses isolated from individuals with respiratory illnesses. Antigenic relationships between these types and others have been demonstrated by neutralization in WI-38 cell cultures and by immunodiffusion. Types 1A and 1b, 2 and 49, 23 and 30, 29 and 44 and others are related. With specific animal antisera, heterotypic antibody titers are minimal. However, following natural infection in man with one of these related types, antibody responses to the other was almost equal to the homotype and could predictably provide cross-protection. Even with inactive T13 rhinovirus vaccine in man, protective levels of neutralizing antibody to T41 were produced. It is clear that the rates of rhinovirus colds would be greatly increased if more than 89 immunologic distinct types actually existed. An effective vaccine could be easily prepared by selection of a few appropriate types, such as 1A, 2, 23, 29 and 31.