The plant defence activator, Acibenzolar-S-methyl [benzo (1,2,3) thiadiazole-7-carbothioic acid-S-methyl ester, ASM] was assayed on tomato seedlings for its ability to induce resistance against Botrytis cinerea, the causal agent of gray mold in tomato. Pre-treatment of plants with ASM reduced the severity of the disease as well as the growth of the mycelium in plants. In ASM treated plants, reduction in disease severity (up to 55%) was correlated with suppression of mycelia growth (up to 46.5%) during the time course of infection. In plants treated with ASM, activities of peroxidase were determined as markers of resistance. Applications of ASM induced Progressive and significant increase of the enzyme in locally treated tissues. Such responses were expressed earlier and with a much higher magnitude when ASM-treated seedlings were challenged with the pathogen, thus providing support to the concept that a signal produced by the pathogen is essential for triggering enhanced synthesis and accumulation of the enzymes. No such activities were observed in water-treated control plants. Therefore, the slower symptom development and reduction in mycelium growth in ASM treated plants might be due to the increase in activity of oxidative and antioxidative protection systems in plants.