Effervescent lozenges containing 10 mg of zinc acetate were evaluated as a treatment of upper respiratory tract infections in a double-blind randomized trial by using a placebo which was indistinguishable to most observers in taste and appearance from the active material. Of the 70 treatment courses used by 55 individuals in 34 families, 63 (33 zinc and 30 placebo) were considered evaluable, in that the volunteer used the medication at least four times daily for at least 3 days, the average utilization being 5.4 days at an average dose of six lozenges daily. Six users of zinc reported nausea (versus no placebo users), and eight reported an unpleasant taste or aftertaste (versus one placebo user). No benefit was observed among the users of zinc acetate. The mean duration of symptoms in users of the zinc was 12.1 days, compared with 7.7 days in those who used the placebo. Nor was any beneficial effect of zinc evident among the four zinc-treated versus the two placebo-treated individuals from whom rhinovirus was grown.