Ninety rhinovirus types and a prime strain of RV 22 were compared in reciprocal serum neutralization tests. Neutralization was observed for each of the 91 homologous virus-serum mixtures. A single instance of reciprocal neutralization between two rhinovirus serotypes was noted. This reciprocal relationship, between RV types 29 and 44 was not altered by absorption with human liver powder. It is suggested that the relationship between RV types 29 and 44 may be a prime strain relationship similar to that described for RV type 22.