Conditions are described whereby the ADP-ribosylation (from NAD+) of reticulocyte elongation factor EF-2, catalyzed by diphtheria toxin, is essentially complete and whereby the reverse of this process may be carried out with recovery of 60--70% of the original EF-2 activity. Both reactions proceed well at room temperature. The reverse reaction is much slower than the ADP-ribosylation process and requires high nicotinamide concentrations. For the reverse reaction to occur at a significant rate it is necessary to lower the pH to 6.5 (from the 7.5 used for the forward reaction). NAD+ covalently linked to agarose may replace NAD+ in the diphtheria toxin reaction. The characteristics of this reaction are similar to those of the reaction employing free NAD+ except that the velocity is reduced and the concentration of NAD+ moieties greatly increased. NAD+ immobilized on agarose through the C-8 of the adenine ring is a superior substrate compared with NAD+ linked to agarose via its periodate-oxidized ribose moieties. Preliminary experiments indicate that reversal of this latter reaction with recovery of biological activity may be possible.