The therapeutic effect of intranasally administered enviroxime was tested against naturally occurring common colds. The double-blind evaluation was carried out in Tecumseh, Mich., during a period when rhinoviruses are usually the principal pathogen. Rhinovirus transmission followed the typical pattern during this period of study. Although there were trends indicating greater therapeutic effectiveness for enviroxime when certain nasal symptoms were considered, there were no consistent statistically significant differences between treated and untreated groups. Results were unchanged when illnesses in different periods or associated with rhinovirus isolation were examined. It was concluded that no therapeutic effect of enviroxime was demonstrated.