The effects of joint action of SO2 and HF on the yield and quality of wheat and barley were studied by exposing them to combinations of < 13, 130 or 267 μg m-3 SO2 and 0·03 or 0·38 μg m-3 HF in open top chambers for 90 days. At the concentrations used, SO2 had greater effects than HF. All responses were marked by compensatory changes. The treatments had no effect on wheat yield, although SO2 reduced shoot weight. SO2 increased the growth and yield of barley, and HF or SO2 increased the grain protein concentration of barley and wheat. The effects of mixtures of SO2 and HF were complex, but often antagonistic, as the addition of HF counteracted the effect of SO2 alone.