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Tomato cultivars Zuiko (ZK), resistant to TMV causing top-necrosis by infection with TMV, Fukuju No. 2 (F2), susceptible to TMV causing mosaic, strains GCR 236, GCR 237, GCR 254, and GCR 267, resistant to TMV causing no symptoms, were used. They were tested whether or not the induced resistance does occur by changing temperatures during incubation following inoculation, by heating tomato seedlings as a shock at 45°C for 10 min immediately after inoculation, by introducing dyes, acridine orange (AO), azure B (AB), and pyronine B (PB), into tomato seedlings, and by introducing the infected tomato extract.At 25°C, TMV concentration in ZK was 2% of that in F2, whereas it reached almost 60% at 20°C, showing that ZK was not resistant at a low temperature below the optimum for tomato growth. Heat shock induced the reduced amount of TMV in ZK as compared with non-treatment, but did not in GCR 236, GCR 237, and GCR 267. Thus, ZK alone showed an induced resistance by this treatment. ZK, GCR 236, GCR 237, and GCR 254, all showed a decrease in TMV concentration by introducing dyes through root, especially by 10 ppm AO. However, F2 cultured in 10 ppm AO showed no alteration in TMV concentration comparing with that cultured in water. When the extract obtained from non-infected ZK which was cultured in 10 ppm AO, was mixed with TMV and the mixture was inoculated on the leaves of bean (Phaseolus vulgaris), AO absorbed-ZK extract showed no inhibitory activity to TMV infection. These facts indicate that AO did not directly inactivate TMV, but induced a highly resistance in ZK except F2. F2 which had absorbed the infected F2 extract, reduced TMV concentration, but that absorbed non-infected F2 extract or infected ZK extract did not so much. ZK did not reduce TMV concentration by absorbing F2 extract or non-infected ZK extract, but did a little by absorbing infected ZK extract.From these results, induction of resistance in tomato cultivars caused by the environmental factors is discussed.