The goal of our current research was to investigate the antioxidative effects of methanolic extracts from different parts of adlay seed and their antiproliferative activity in malignant human cells. The methanolic extracts from different parts of adlay seeds were from the hull (AHM), testa (ATM), bran (ABM), and polished adlay (PAM). AHM exhibited greater capacity to scavenge superoxide anion radicals in the PMS-NADH system than ATM, ABM, or PAM. The scavenging capacities of AHM and ATM on hydrogen peroxides were about 20% at a dose of 250 microg/mL. Using the method of deoxyribose degradation to assess damage caused by hydroxyl radicals, AHM was found to inhibit damage in deoxyribose at a higher concentration. However, ATM, ABM, and PAM exhibited prooxidative activity at the same concentration. The inhibitory effect on enzymatic oxidation of xanthine to uric acid was found to follow the order AHM > ATM =. ABM. However, PAM was inactive. All test samples were positive for inhibition of TPA-induced free radical formation on neutrophil-like leukocytes and were found to follow the order AHM > ATM > ABM > PAM. When human histolytic lymphoma U937 monocytic cells were exposed to tert-butyl hydroperoxide, AHM protected the cells against the cytotoxicity caused by tert-butyl hydroperoxide. In addition, AHM exhibited antiproliferative activity against human histolytic lymphoma U937 monocytic cells in a dose-dependent manner. The antiproliferative properties of AHM appear to be attributable to its induction of apoptotic cell death as determined by flow cytometry. These results show that AHM displays multiple antioxidant effects and induces apoptosis of malignant human cells.