We have analysed, by PCR using consensus primers followed by sequencing, 12 human rhinoviruses (HRVs) in a genomic region including that corresponding to the immunogenic site NIm-II. Together with published information, 21 sequences are available for comparison. In the region analysed, which encodes 112 amino acids, the majority (18) of the serotypes exhibited at least 70% amino acid identity to one another and some serotypes are very closely related. These include HRV-36, -58 and -89, known to exhibit antigenic cross-reactivity, which were shown to differ at only three amino acid positions. Three serotypes, HRV-3, -14 and -72, share at least 84% identity with one another but are less than 66% identical to the majority group. Interestingly, membership of these two molecular clusters correlates with the groupings determined by sensitivity to antivirus drugs, suggesting that they reflect a fundamental division of HRVs. In contrast, there is no correlation with receptor grouping, since the majority group contains members belonging to both HRV receptor groups.