Cholera toxin (CT) produced by Vibrio cholerae causes the devastating diarrhea of cholera by catalyzing the ADP-ribosylation of the alpha subunit of the intestinal Gs protein (Gsalpha), leading to characteristic water and electrolyte losses. Mammalian cells contain ADP-ribosyltransferases similar to CT and an ADP-ribosyl(arginine)protein hydrolase (ADPRH), which cleaves the ADP-ribose-(arginine)protein bond, regenerating native protein and completing an ADP-ribosylation cycle. CT-catalyzed ADP-ribosylation of cell proteins can be counteracted by ADPRH, which could function as a modifier gene in disease. Further, our study demonstrates that enzymatic cross talk exists between bacterial toxin ADP-ribosyltransferases and host ADP-ribosylation cycles. In disease, toxin-catalyzed ADP-ribosylation overwhelms this potential host defense system, resulting in persistence of ADP-ribosylation and intoxication of the cell. Mono-ADP-ribosylation is a reversible modification of proteins with NAD:arginine ADP-ribosyltransferases and ADP-ribosylarginine hydrolases (ADPRH) catalyzing the opposing arms of an ADP-ribosylation cycle. The ADPRH cDNA had been cloned from human, rat, and mouse tissues and high levels of mRNA were found in brain, spleen, and testis. Human ADP-ribosylhydrolase 1 (hARH1, ADPRH) cleaves the glycosidic bond of ADP-ribose attached to an Arg residue of a protein.