Measurements of the initial rate of ADP-ribosylation of elongation factor 2 (EF-2) catalyzed by Fragment A from diphtheria toxin support a sequential mechanism and suggest that the reaction proceeds through a central ternary complex involving Fragment A and the substrates, EF-2 and NAD. The Michaelis constants for EF-2 and NAD are 0.15 and 1.4 muM, respectively. As determined by equilibrium gel permeation, EF-2 does not bind Fragment A significantly, alone or in the presence of adenine, ADPribose, nicotinamide or NADH. Based on these and earlier results, we propose an ordered sequential mechanism for the reaction; the sequence of binding of substrates is NAD, followed by EF-2.