The activity of DNA polymerase alpha in splenocytes of 24-month-old mice was about 35.6% lower than that of 3-month-old mice. Aged mice were intraperitoneally administered Ganoderma polysaccharides (GL-B) once a day for 4 days and then the activity of the enzyme was assessed. The results showed that GL-B at doses of 25 and 50 mg/kg-1 enhanced the activity of the enzyme in aged mouse splenocytes by 44.0 and 58.8% respectively. In addition, the mixed lymphocyte response to alloantigen, automatic proliferation and IL-2 production of splenocytes in aged mice declined as compared with that in young adult mice. GL-B (50, 100, 200 micrograms.ml-1) was found to restore those parameters to the levels of that of young mice in vitro.