Bioassay-guided fractionation using an assay to monitor DNA polymerase beta inhibition led to the isolation of-the inhibitor, mispyric acid (I), a monocyclic triterpenoid with a novel skeleton from Mischocarpus pyriformis Its. structure, including the relative stereochemistry, was established ori the basis of HRMS and ID and 2D NMR. spectroscopic methods. Mispyric acid inhibited DNA polymerase beta with an IC50 of 20 mu M in the presence of bovine serum albumin (BSA) and 14 mu M in the absence of BSA, consistent with the possibility that 1 may be of utility in vivo. This possibility was further supported by the finding that compound 1 successfully potentiated the cytotoxic action of bleomycin in cultured cells, reducing the number of viable cells by nearly 50% when employed at 50 mu M concentration in the presence of an otherwise nontoxic (75 nM) concentration of bleomycin. Finally, the novel structural framework of 1 suggests Mat its biosynthesis may obtain via a novel biogenetic pathway.