Two series of field experiments were carried out to test the effects of aerial pollutants, sulphur dioxide and fluoride, on the growth and yield of cereals, where there is no visible damage to the plant. The first series, involving barley grown in filtered plastic-covered hooped channels, proved unsatisfactory. The growing conditions of temperature, humidity and light intensity differed from those outside. Modifications were made and an open-topped chamber was introduced. Comparisons were made between plants grown in filtered and unfiltered closed-top and open-top chambers and outside. The fluoride content of the leaves was greatest in the open-top chamber without filtration and least in the closed-top chamber with filtration. Sulphur content was highest in the plants grown outside and lowest in the open-top chamber with filtration. Plant growth and grain yields were higher in the filtered chambers. This strongly suggests a significant loss of yield due to a pollutant level which was earlier considered acceptable.