Numerous studies argue that salicylic acid (SA) is an important component of the plant signal transduction pathway(s) leading to disease resistance. The discovery that the SA-binding protein is a catalase, whose activity is blocked by SA, led to the proposal that one of SA's modes of action is to inhibit this H2O2-degrading enzyme and thus elevate H2O2 levels. To test this model, an attempt was made to mimic the action of SA by reducing the synthesis of catalase using antisense RNA technology. Analyses of transgenic tobacco plants that expressed the tobacco catalase 1 (cat1) or catalase 2 (cat2) gene in an antisense orientation indicate that there is no correlation between modest to high levels of reduction in catalase activity and activation of plant defenses such as pathogenesis-related (PR)-1 protein synthesis. However, three independent antisense catalase transgenic plants (ASCAT1 Nos 16, 17, and 28), which exhibited the most severe reduction in catalase activity (approximately 90% or more), developed chlorosis or necrosis on some of their lower leaves. These same leaves accumulated very high levels of PR-1 proteins and showed enhanced resistance to tobacco mosaic virus. Necrosis and elevated SA, which appear to result from severe depression of catalase levels, may be responsible for the induction of these defense responses.