In this study, we demonstrated selenium (Se) accumulation in Bifidobacterium longum strain (B. longum)and evaluated the effect of Se-enriched B. longum (Se-B. longum) on tumor growth and immune function intumor-bearing mice. Analysis using high-performance liquid chromatography-inductively coupled plasmamass spectrometry (HPLC-ICP-MS) revealed that more than 99% of Se in Se-B. longum was organic, the maincomponent of which was selenomethionine (SeMet). In the in vivo experiments, tumor-bearing mice (n=8) wereorally administrated with different doses of Se-B. longum alone or combined with cyclophosphamide (CTX).The results showed that the middle and high dose of Se-B. longum significantly inhibited tumor growth. WhenSe-B. longum and CTX were combined, the antitumor effect was significantly enhanced and the survival time oftumor-bearing mice (n=12) was prolonged. Furthermore, compared with CTX alone, the combination of Se-B.longum and CTX stimulated the activity of natural killer (NK) cells and T lymphocytes, increasing the levels ofinterleukin-2 (IL-2) and tumor necrosis factor-α (TNF-α), and the leukocyte count of H22 tumor-bearing mice(n=12).