After inhalation by mammals, Bacillus anthracis spores germinate in alveolar macrophages then migrate to lymph nodes where they multiply. The vegetative bacteria excrete the tripartite exotoxin which consists of three polypeptides: protective antigen (PA, 83kD), lethal factor (LF, 90kD) and oedma factor (OF, 89kD). The two components (OF and LF) of the toxin enzymatically modify substrates within the cytosol of the mammalian cells: The OF is an adenylate cyclase that impairs the host defenses through a variety of mechanisms inhibiting phagocytosis. The LF is a zinc dependent protease that cleaves several mitogen activated protein kinase kinases (MAPKK) and causes lysis of macrophages. To intoxicate mammalian cells, the third component of the toxin PA, binds to a ubiquitously expressed cellular receptor, Tumor Endothelium Marker 8 (TEM8). Upon binding to TEM8, PA is cleaved into 20 and 63kD fragments (PA20 and PA63) by furin or furin-like proteases. PA20 dissociates into medium and allows the PA63 fragment to heptamerize and bind LF and OF of the toxin. The resulting complex of PA63 fragment with EF and/or OF binds to PA-receptor TEM8/ATR and internalized into endosomes followed by translocation of LF and OF into cytosol of the cells. Anthrax lethal toxin produced by the bacterium Bacillus anthracis is the major cause of death in animals infected with anthrax. One component of this toxin, lethal factor (LF), inactivates members of the mitogen-activated protein kinase kinase or MEK family through proteolysis of their NH2 termini. Although LF has been shown to cleave the NH2 termini of select members of the mitogen-activated protein kinase kinase or MEK family, the substrate requirements that determine LF specificity are unknown. Indirect evidence suggests that epitopes distal to the cleavage site are required for LF-MEK interaction.