AbstractClones of eastera white pine (Pinus strobus) selected for their sensitivity to sulfur dioxide (SO2) were fumigated in a controlled environment chamber. Sulfur dioxide alone and in combination with ozone (O3) at concentration 5 parts per hundred million (pphm) of air by volume of each gas for independent 2‐hr exposures was toxic to new needles. Severest injury resulted from a discontinuous fumigation with 5 pphm O3and 5 pphm SO2for 2 hr in separate fumigations, followed 24 hr later by a 2‐hr exposure to a mixture of these gases, each at 5 pphm. The rapidity of lesion development and total injury to new needles were less following a single 2‐hr fumigation with an O3‐SO2mixture than when a single 2‐hr fumigation with SO2alone was used. Ozone and SO2reacted to reduce injury. These plants were not injured by O3used alone. New needles receiving only filtered air or new needles on resistant plants were not visibly affected by any treatment. Acute injury induced by the O3‐SO2mixture developed differently from injury induced