Although sulfur dioxide is the most common and usually the most abundant polluting agent, experiments with this gas have shown that, under the conditions employed, there is no effect on yield unless the concentration is high enough to cause leaf injury. S23 ryegrass was grown in beds in greenhouses. Treatment with purified air gave, with one exception, a dry weight considerably higher than treatment with polluted air. No sign of leaf damage could be detected in the greenhouse with polluted air. This fact would seem to indicate that pollution decreases the growth rate, even in the absence of visible injury.