1. A polysaccharide-enriched fraction (CPPS) was prepared from Codonopsis pilosula root extract utilizing a procedure that entailed extraction with aqueous buffer and precipitation with ethanol. 2. After administration of CPPS in drinking water to C57BL/6 mice at a dosage of 10 mg/L for 4 weeks, the splenocytes exhibited lowered mitogenic responses to Concanavalin A (ConA) and lipopolysaccharide (LPS). The in vitro production of reactive nitrogen intermediates was inhibited. 3. However, when oral administration of CPPS was prolonged to 8 weeks, there was a potentiation of ConA-stimulated and LPS-stimulated mitogenic responses. 4. When tested under in vitro conditions, CPPS augmented the mitogenic response of splenocytes to ConA. However, there was no effect on the pinocytic activity of mouse macrophages, nor was there any proliferative activity on mouse melanoma B16 cells.