Background and purpose:Maintenance of poly(ADP‐ribose) (PAR) polymers at homoeostatic levels by PAR glycohydrolase (PARG) is central in cell functioning and survival. Yet the pharmacological relevance of PARG inhibitors is still debated. Gallotannin, a complex mixture of hydrolysable tannins from oak gall, inhibits PARG but which of its constituents is responsible for the inhibition and whether the pharmacodynamic properties are due to its antioxidant properties, has not yet been established.Experimental approach:A structure–activity relationship study was conducted on different natural and synthetic tannins/galloyl derivatives as potential PARG inhibitors, using a novel in vitro enzymic assay. Cytotoxicity was assayed in cultured HeLa cells.Key results:Mono‐galloyl glucose compounds were potent inhibitors of PARG, with activities similar to that of ADP‐(hydroxymethyl) pyrrolidinediol, the most potent PARG inhibitor yet identified. When tested on HeLa cells exposed to the PAR polymerase (PARP)‐1‐activating compound 1‐methyl‐3‐nitro‐1‐nitrosoguanidine (MNNG), 3‐galloyl glucose weakly inhibited PAR degradation. Conversely, the more lipophilic, 3‐galloyl‐1,2‐O‐isopropylidene glucose, despite being inactive on the pure enzyme, efficiently prolonged the half‐life of the polymers in intact HeLa cells. Also, PARG inhibitors, but not radical scavengers, reduced, in part, cell death caused by MNNG.Conclusions and implications:Taken together, our findings identify mono‐galloyl glucose derivatives as potent PARG inhibitors, and emphasize the active function of this enzyme in cell death.British Journal of Pharmacology (2008) 155, 1235–1249; doi:10.1038/bjp.2008.370; published online 22 September 2008