The present study is aimed to examine the possibility of enhancement of natural killer (NK) cell activity in aged C57BL/6 and C3H mice using MGN-3, a modified arabinoxylan from rice bran. Intraperitoneal injection of MGN-3 (10 mg kg(-1) per day) caused a remarkable increase in the peritoneal NK activity as early as 2 days (35.2 lytic units), and the level remained elevated through day 14. The control aged mice had a level of 5.8 lytic units. Enhancement in NK activity was associated with an increase in both the binding capacity of NK cells to tumour targets and in the granular content as measured by BLT-esterase activity. Treatment did not alter the percentage of peritoneal NK cells. Data showed that peritoneal macrophages inhibit NK activity. In conclusion, MGN-3 enhances murine NK activity of aged mice and may be useful for enhancing NK function in aged humans.