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Stimulation of active oxygen metabolism occurs during the early stages of interactions involving bacteria and plant cell suspensions. Although many cellular processes are known to affect active oxygen metabolism in plants, it is not known which of these factors affect active oxygen levels during plant-bacteria interactions. Extracellular peroxidases have been shown to participate in both the production and utilization of active oxygen species such as H,O, and superoxide. Catalase and other scavenging mechanisms also affect the overall level of active oxygen. In this study the luminol-dependent chemiluminescent reaction previously used to measure H,O, levels in suspension cells was modified to allow the assay of both peroxidase and H,O,-scavenging activity. The early stages of the interactions between tobacco (Nicofiana fabacum) and Pseudomonas syringae pv syringae, as well as between soybean (Glycine max) and P. syringae pv glycinea, were investigated. This method of monitoring peroxidase and H,O,-scavenging activity proved to be rapid, sensitive, and nonintrusive, allowing the processing of multiple samples using intact cells or cell-free preparations. The results from the study demonstrate that the scavenging activities can be significant and must be considered when studying active oxygen production in biological interactions.As the study of plant-microbe interactions has become more molecular, there has been an increased use of specialized model systems to allow the study of specific phenomena. Studies of the bacteria-induced HR found that inoculation with incompatible species produces rapid plant cell death in cell suspensions similar to that found during the HR in whole plant tissues (Baker et al., 1993a;Baker and Mock, 1994). The use of suspension cells in addition to whole plants has allowed identification of early plant responses, which precede hypersensitive cell death (Atkinson et al., 1985b;Keppler et al., 1989;Baker et al., 1991Baker et al., , 1993a. One of these responses is the production of active oxygen. The response consists of a rapid, transient production a few minutes after addition of the incompatible bac-