The therapeutic effect of intranasal 2-amino-1-(isopropylsulphonyl)-6-benzimidazole phenyl ketone oxime (enviroxime) against human rhinovirus type 9 was evaluated in a double-blind, placebo-controlled volunteer trial. Enviroxime given 6 times a day was well tolerated, producing a reduction in clinical evidence of infection which coincided with the start of medication (44 h after virus challenge). Although there was a statistically significant reduction in clinical score in the enviroxime group on day 5 after virus challenge, reductions in total clinical score accumulated during the trial and reductions in the quantity of nasal secretions were not significant. A separate analysis was performed on data from volunteers who did not have symptoms when medication began (and who might have been expected to benefit more from enviroxime therapy). No apparent enhancement of the effects of enviroxime could be demonstrated in this group.