Since the discovery of generation of O2 - in plant, many evidence for the oxidative burst (OXB) has been accumulated in various combinations of plant and pathogen or elicitor systems. O2 - generating system responsible for the OXB was coupled with oxidation of reduced nicotinamide adenine dinucleotide phosphate (NADPH) in microsomal fraction isolated from sliced aged potato tuber slices which were treated by hyphal wall components elicitor from Phytophthora infestans (HWC). We developed new assay method for quantitative measurement of oxygen radical O2 - by using electron spin resonance (ESR) analysis during elicitorinduced OXB on the surface of plant tissues. The ESR analysis using an O2 - trapper, Tiron (1,2-dihydroxy-3,5- benzenedisulfonic acid), provided a convenient assay for detecting only O2 - during elicitor-induced OXB producing various active oxygen species (AOS) on plant tissue surface. Tiron was oxidized to Tiron semiquinon radical by O2 -. Quantity of the radical signal was measured by specific spectra on ESR spectroscopy. The level of O2 - was high in from surface of potato tuber tissue treated with hyphal cell wall elicitor (HWC) from Phytophthora infestans. There was no secondary OXB induction by H2O2 treatment in plant.