We describe here a cDNA:RNA hybridization system for the study of human rhinoviruses. We have constructed an M13 probe from the 5\' end of the genome of rhinovirus 14 (HRV-14) and used this to detect directly viral RNA. Of the 56 human rhinoviruses so far investigated 54 or 96.4% gave clearly positive hybridization signals. However, the strength of this signal depended very much on the molecular relationship of these viruses. Thus, HRV-3, 4, 17, 72, and, to a slightly lesser extent, HRV-2, 6, 9, 13, 19, 31, 42, 49, 64, and 69 appear to be closely related to HRV-14 whereas HRV-5, 7, 8, 16, 32, 40, 45, 55, 56, 63, 80, 82, and 85 appear to be relatively divergent. Further, evidence is provided in this study that indicates that it would be feasible to use cDNA probes to detect human rhinoviruses in nasal washings. However, the sensitivity of detection was clearly affected by both the inclusion of inhibitors of endogenous RNase activity in the RNA extraction mixture and also in the method of extracting the viral RNA. From reconstruction experiments in nasal washings and under optimal conditions, we can detect virus at 10(2.8) TCID50/ml.