To develop a screening system for plant defense activators, which are novel substances that protect plants by enhancing their inherent disease-resistance mechanisms, we utilized a GUS histochemical staining assay using promoters of the defense-related genes, PR-1 and PDF1.2. We can perform about 1,000 screenings per week per person by this high-throughput screening method. This GUS assay for plant defense activator candidates was evaluated by QRT-PCR analysis to elucidate the functions of the plant defense activators in detail. In the present preliminary screening, we evaluated two hundred chemicals chosen at random. Some chemicals induced GUS activity in a PR-1 promoter::GUS transformant, i.e., abietic acid, allose, glycine, and thymol. The induction of PR-1 expression by the treatments with these chemicals was confirmed using QRT-PCR. The foliar treatment with abietic acid 1 d prior to inoculation with the fungal pathogen Colletotrichum higginsianum led to a significant reduction of necrotic surface area compared with distilled water treated controls, as observed 6 d after inoculation. These results suggest that this GUS histochemical staining assay is an effective and available screening system for plant defense activators.