Codonopsis lanceolata has long been used as a seasonal food and as a traditional tonic medicine with anti-inflammatory and anti-oxidation properties. The present study investigated the in vitro effect of Codonopsis lanceolata extracts on immune function in mice. After preparing a single cell suspension splenocyte proliferation was determined by the MTT(3-[4,5-dimethylthiazol-2-y]-2,5-diphenyl terazolium bromide) assay. The cytokines IL-1${eta}$, IL-6, and TNF-$alpha$ were not secreted by macrophages stimulated with or without LPS as determined by an ELISA cytokine kit assay. After a 48-hr incubation with the mitogens ConA or LPS there was an increase in splenocytes proliferation and in the production of IL-1${eta}$, IL-6, and TNF-$alpha$ in the suspensions supplemented with 50, 100, 250, 500 ${mu}g/m{ell}$ Codonopsis lanceolata water extract. The results suggest Codonopsis lanceolata water extract may enhance immune function by regulating splenocyte proliferation and stimulating cytokine production.