The growth of soybean was inhibited by exposure to 490 .mu.g O3/m3 (25 pphm [parts per hundred million]) and 665 .mu.g SO2/m3 (25 pphm), singly and in combination, when plants were exposed for 4 h 3 times/wk for 11 wk. The main effects of O3 were a reduction of shoot, root and plant dry weight measured at 5, 7, 9 and 11 wk. The main effects of SO2 were a reduction of shoot dry weight at 7 wk and total plant growth at 11 wk. SO2 contributed to the reduced growth in soybean in the absence of visible SO2 injury. The effect of SO2 and O3 in combination on soybean growth was only additive. Treatments containing O3 reduced the numbers and dry weight of root nodules of soybean, compared with treatments without O3.