The X-ray structure of the catalytic domain of Pseudomonas aeruginosa exotoxin A (PE24) has recently been solved to high resolution, facilitating studies on the interaction of PE24 with its target substrate, eukaryotic elongation factor-2 (eEF-2). PE24 exhibits mono-ADP-ribosyltransferase (ADPRT) activity in a mechanism that has been proposed to feature a nucleophilic attack by the diphthamide residue (nucleophile) of eEF-2 on the C-1 of the nicotinamide ribose of NAD ุ . The interaction of wheat germ eEF-2 with PE24 was studied by employing an enzyme-linked immunosorbent assay (ELISA), devised to assess protein-protein interactions. It was shown that the proteins associate with each other only in the presence of the enzyme's nucleotide substrate, NAD ุ , and exhibit a dose-dependent association that is saturable. The apparent dissociation constant (K d ) for this proteinprotein interaction is 50 nM and is salt-dependent. The association is maximal at low ionic strength and is progressively weaker at higher salt concentrations, which corroborates previous findings on the salt dependence of ADPRT activity for this toxin. This finding suggests that the sensitivity of ADPRT activity toward high salt resides in the interaction between the catalytic domain of the toxin and eEF-2. A major product of the glycohydrolase activity of PE24, nicotinamide, inhibits the binding between PE24 and eEF-2 with an ID 50 of 20 M. The naturally occurring, noncatalytic mutant of PE24, H426Y, did not bind eEF-2 in the ELISA, verifying that His 426 is located at the center of the eEF-2 binding site within ETA.