Bioassay-guided fractionation of an active methyl ethyl ketone extract of Tetracera boiviniana, using a sensitive assay to monitor DNA polymerase beta inhibition, resulted in the isolation of three known triterpenoids, betulinic acid (1), 3-cis-p-coumaroyl maslinic acid (2), and 3-trans-p-coumaroyl maslinic acid (3). Compounds 1-3 inhibited DNA polymerase beta with IC50 values of 14, 15, and 4.2 microM in the presence of bovine serum albumin (BSA) and 6.5, 7.5, and 2.0 microM in the absence of BSA, respectively. Further, compounds 1-3 potentiated the effects of bleomycin in cultured P-388D1 cells.