Two new flavonoids - 3'-formyl-4',6'-dihydroxy-2'-methoxy-5'-methylchalcone (FMC) and (2S)-8-formyl-5-hydroxy-7-methoxy-6-methylflavanone (FMF) - isolated from the buds of Cleistocalyx operculatus, were investigated for their antioxidant and anticancer activity. Total antioxidant activity and reducing ability were measured. 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical and superoxide anion radical scavenging assays were carried out to evaluate the antioxidant potential of the two compounds. The antioxidant activity of the two compounds increased in a concentration-dependent manner. FMC and FMF at a concentration of 500 microM inhibited lipid peroxidation by 64.3 +/- 2.5% and 60.3 +/- 2.3%, respectively, an antioxidant activity approximately similar to that of 500 microM alpha-tocopherol (66.3 +/- 2.5%). Similarly, the effect of FMC and FMF on reducing power increased in a concentration-dependent manner. In DPPH radical scavenging assays, the IC50 values of FMC and FMF were 50.2 +/- 2.8 microM and 75.8 +/- 2.5 microM, respectively. Moreover, FMC and FMF scavenged the superoxide generated by the phenazine methosulfate (PMS)/reduced beta-nicotinamide adenine dinucleotide (NADH) nitroblue tetrazolium (NBT) system, with IC50 values of 56.3 +/- 2.3 microM and 317.5 +/- 2.9 microM, respectively. The anticancer activity of the two compounds were determined in five human cancer cell lines, SMMC-7721 (liver cancer), 8898 (pancreatic cancer), K562 (chronic leukaemia), HeLa (tumour of cervix uteri) and 95-D (high metastic lung carcinoma). FMC and FMF showed broad-spectrum anticancer activity against all the human cancer cell lines tested. The results obtained in the current study indicate that the two flavonoids could be a potential source of natural antioxidant and anticancer agents. To our knowledge, this is the first report on bioactivity of FMC and FMF.