ADP-ribosylation of arginine residues in proteins may be involved in cell adhesion and is crucial for the action of cholera toxin and E. coli heat-labile enterotoxin, agents involved in the pathogenesis of cholera and traveller's diarrhoea, respectively. ADP-ribosylation is reversed by ADP-ribosylarginine hydrolases, which cleave the ADP-ribose-arginine bond. ADP-ribosylarginine hydrolases from a variety of mammalian species and tissues were isolated, and the coding regions for the hydrolases were cloned and expressed. The availability of this new hydrolase cDNA and expression system provides a novel molecular approach for studying the role of ADP-ribosylation in cell function. The gene products may be useful in treating or preventing a variety of bacterial diseases, including cholera, that appear to be mediated via ADP-ribosylation.