Bioassay-guided fractionation of the ethyl acetate extract of Virola sp. (Myristicaceae) using the Sc7 yeast strain resulted in the isolation of the weakly cytotoxic biochanin A (an isoflavone compound). The bioassay of three mutant yeast strains (1138, 1104, and 1353) directed the isolation of DNA-damaging alkaloids from Solanum hostmannii. These alkaloids were identified as verazine and (20R) epimer verazine. In addition, three oxoaporphine alkaloids were isolated from the bark of Papualthia sp. (Annonaceae). Oxocrebanine and atherospermidine showed DNA-damaging activity whereas liriodenine had cytotoxic activity. / Master of Science