Reactive oxygen species such as hydrogen peroxide (H2O2) have taken center stage as bona fide second messengers in various signaling pathways. Here, we report the synthesis, metabolic fate, and effectiveness in modulating such pathways of a Tat-catalase conjugate. Incubation of L2 cells with Tat-catalase greatly increased cell-associated enzymatic activity, reaching close to a plateau by 30min. The cell-associated catalase activity and antibody-detectable Tat-derivatives declined over time after changing medium, although still remaining at significantly higher levels than baseline even at 4h. While most cell-associated Tat-catalase was apparently tightly attached to the cell surface, a small fraction entered the cells as the proteasome inhibitor MG-132 slightly prevented the disappearance of the enzyme. Tat-catalase, either membrane-bound or intracellular, but not native catalase, inhibited serum-induced Elk phosphorylation and anisomycin- and/or MG-132-induced ERK phosphorylation, suggesting the involvement of H2O2. Thus, Tat-catalase should be a useful tool to dissect H2O2-dependent events in signaling pathways. © 2003 Elsevier Science (USA). All rights reserved.