Studies were undertaken to evaluate ozone (O(3)) for induction of resistance against Cucumber mosaic virus in Lycopersicon esculentum cv. PKM1 (tomato) plants. Callus induced from tomato leaf explants on Murashige & Skoog's (MS) medium supplemented with benzyladenine (8.82 microM) were treated with different concentrations of ozone T(1), T(2), T(3) and for control (C), filtered air was supplied. Regeneration of shoots was obtained by culturing ozone treated calli on MS medium containing 17.3 microM benzyladenine. The frequency of regeneration of tomato plants from the callus were T(1)=79%, T(2)=61%, T(3)=42%, but for control 90% regeneration was obtained. Regenerated plants were rooted in half strength MS medium supplemented with 10 microM indole-butyric acid and successfully acclimatized. The plants regenerated from ozone treated callus are referred to as T(1), T(2) and T(3) plants, which hold remarkably increased soluble phenolic content compared to the control plants. All the plants were challenged by mechanical inoculation with Cucumber mosaic virus, shows disease incidence ranged from T(1)=32%, T(2)=56%, T(3)=78% and C=94%. Remarkable increase in activities of salicylic acid (SA), phenylalanine ammonia-lyase (PAL) and peroxidase (POX) were detected after Cucumber mosaic virus inoculation, in foliar extracts of T(1) plants than T(2) and T(3), compared to the control plants.