A controlled trial was undertaken to test I.R.S. 19 (a commercial intranasal spray) versus placebo in the prevention of acute respiratory diseases (ARD) in 825 maternity-school children in three cities; another control group of 327 children received neither I.R.S. 19 nor placebo. The spraying was done twice a day for a total of 20 spraying days in each child; sprayings were interrupted on weekends and during absence, the mean spraying period being 34 calendar days. During the 6-month study (1 November to 30 April) the children were monitored for ARD morbidity causing absence from school. A total of 1,585 such ARD cases occurred; their etiology was not investigated. The indices evaluated were: total duration of ARD-associated absence, ARD incidence, and mean duration of one illness. With the administration schedule used, I.R.S. 19 did not, in an overall evaluation, surpass placebo in any of these indices in either normal children or a subgroup of children with presumed enhanced ARD susceptibility.