A novel alpha-tetralonyl derivative, juglanone, was isolated from the fresh unripe fruits of Juglans mandshurica. Its structure was determined by spectroscopic analyses and from chemical evidence. Juglanone exhibited significant antioxidant activity in assays for 1,1-diphenyl-2-picrylhydrazyl radical scavenging activity and superoxide dismutase-like activity with IC(50) values of 10.1 and 9.0 microM, respectively. It also showed moderate cytotoxic activity against HL-60 human myeloid leukemia with an IC(50) value of 19.7 microM.