Achyranthes bidentata polysaccharides (ABP), extracted from the root of Achyranthes bidentata, induced interleukin-1 (IL-1) synthesis as well as tumor necrosis factor-alpha (TNF-alpha) synthesis and secretion from thioglycolate-primed mouse peritoneal macrophages in vitro. ABP 100-800 micrograms.ml-1 enhanced both synthesis and release of IL-1 when stimulated by lipopolysaccharides (LPS) (5 micrograms.ml-1), but had no significant influences on synthesis and release of TNF-alpha induced by LPS (10 micrograms.ml-1). Studies on IL-1 and TNF-alpha production induced by ABP (200 micrograms.ml-1) alone or plus LPS showed that peak levels of IL-1 release reached at 24 h and that of TNF-alpha release at about 2-6 h after incubation. Peritoneal macrophages from mice ip ABP 25 and 50 mg.kg-1.d-1 x 5 d produced much more IL-1 than those from control group. Peritoneal macrophages from ip ABP 100 mg.kg-1.d-1 x 6 d alone released more TNF-alpha vs control group, and showed a synergetic action with LPS (10 micrograms.ml-1), which was as strong as the positive control agent BCG. These results provide an explanation for the immunopotentiating effect of ABP.