The ability of anti-light chain antibodies to block the endoprotease activity of the toxin was determined using a FRET assay. The extent of catalytic activity was quantified (arbitrary fluorescene units; AFU) using substrate solution alone (SSA), or substrate solution in the presence of progenitor toxin complex, pure neurotoxin, or recombinant light chain (LCA). As shown in the Figure, all three protein preparations possessed catalytic activity. In addition, anti-light chain antibodies (10, 20 or 30 microliters) produced dose-dependent reductions in catalytic activity of all three preparations.