Flowering plants of 5 cultivars of petunia (Petunia hybrida Vilm.), exposed to 2.8 ppm SO2 (16 hours) under controlled environmental conditions, exhibited variation in SO2 sensitivity based on the degree of water-soaked necrotic leaf lesions. The range of SO2 absorption rates was ± 14% of the mean for the 5 cultivars; however, there was a 2-fold difference in sensitivity among the cultivars, indicating that variation in SO2 absorption plays only a limited role in determining the genetic differences involved in the reaction of petunia to SO2. Detached leaves from all cultivars were equally sensitive to a 10 mM Na2SO3 solution. The injury response of leaf discs to Na2SO3 solutions was monitored by chlorophyll extracts. Although the cultivars varied in sensitivity, the ranking differed from that observed for whole plants to SO2. Callus cultures grown on Murashige and Skoog (MS) medium + naphthaleneacetic acid (NAA) at 2.0 mg/liter + ben-zylamino purine (BA) at 0.5 mg/liter were treated with Na2SO3. Their viability, when evaluated by triphenyl tetrazolium chloride (TTC), yielded a sensitivity ranking of the same cultivars which differed from that obtained with both leaf discs and whole plants. No differences in Na2SO3 sensitivity were observed for callus cultures of the cultivars cultured on MS + 1.0 mg/liter 2,4-dichlorophenoxyacetic acid (2,4-D). The dissimilar injury responses to SO3=/HSO3− in different tests on explants of the same cultivars indicate that at each level of plant organization sensitivity to Na2SO3 is determined by different morphological and physiological factors from those which specify whole plant reaction to SO2.