We have completed production in rabbits of potent antisera to the 90 classified rhinovirus serotypes by using methods previously described (M. K. Cooney and G. E. Kenny, Proc. Soc. Exp. Biol. Med. 133:645-650, 1970). Systematic testing by neutralization tests has revealed significant numbers of cross-relationships among rhinovirus types, some of which have already been reported. Herein, our observations are compared with cross-reactions reported in National Institutes of Health reference guinea pig antisera. Also, original rhinovirus isolates, representing serotypes known to be antigenically related to other rhinoviruses, were tested against rabbit antisera to the related serotypes. These tests revealed extensive antigenic variation among isolates identified as rhinovirus 12:78 or 36:58, which are reciprocally related pairs, 41, reciprocally related to 13, and 67, which is related to both 9 and 32. If the rhinovirus serotypes were grouped according to antigenic relationships, 50 types could be included in 16 groups.