In Arabidopsis thaliana, one of the very early immune-related responses induced after elicitor perception is the oxidative burst, i.e., reactive oxygen species (ROS) generation including superoxide anion and hydrogen peroxide (H₂O₂). ROS production plays different roles in a wide range of biotic and abiotic stress responses, including the closure of stomata and the regulation of cell expansion. In particular, elicitor-induced H₂O₂ is produced mainly by the membrane localized NAD(P)H oxidases RESPIRATORY BURST OXIDASE HOMOLOGUE D and F. In this protocol, we describe a simple and reproducible luminol/peroxidase-based assay to detect and evaluate immunity-related accumulation of H₂O₂ produced in Arabidopsis leaf discs treated with immunity elicitors, such as oligogalacturonides (OGs), flagellin (flg22) or the elongation factor-thermo-unstable (EF-Tu - elf18). This method is based on the detection of the luminescence released by excited-luminol molecules generated after the horseradish peroxidase (HRP)-catalyzed oxidation of luminol molecules in the presence of H₂O₂. Levels as well as duration of the luminescence are proportional to the amount of H₂O₂ produced by elicited leaf discs.