Active oxygen species (AOS) are believed to have important roles in plants in general and in plant—pathogen interactions in particular. They are believed to be involved in signal transduction, cell wall reinforcement, hypersensitive response (HR) and phytoalexin production, and to have direct antimicrobial effects. Since current methods are inadequate for localizing AOS in intact plant tissue, most studies have been conducted using cell suspension culture/elicitors systems. 3,3-diaminobenzidine (DAB) polymerizes instantly and locally as soon as it comes into contact with H2O2 in the presence of peroxidase, and it was found that, by allowing the leaf to take up this substrate, in-vivo and in-situ detection of H2O2 can be made at subcellular levels. This method was successfully used to detect H2O2 in developing papillae and surrounding haloes (cell wall appositions) and whole cells of barley leaves interacting with the powdery mildew fungus. Thus, H2O2 can be detected in the epidermal cell wall subjacent to the primary germ tube from 6 h after inoculation, and subjacent to the appressorium from 15 h. The earliest time point for observation of H2O2 in relation to epidermal cells undergoing HR is 15 h after inoculation, first appearing in the zones of attachment to the mesophyll cells underneath, and eventually in the entire epidermal cell. Furthermore, it was observed that proteins in papillae and HR cells are cross-linked, a process believed to be fuelled by H2O2. This cross-linking reinforces the apposition, presumably assisting the arrest of the pathogen.