In the present study, we have tested the effects of 21 salt marsh plants on cell proliferation of mouse immune cells (spleen and thymus) using MTT assay in culture. The methanolic extracts of six salt marsh plants (Rosa rugosa, Ixeris tamagawaensis, Artemisia capillaris, Tetragonia tetragonoides, Erigeron annus, and Glehnia littoralis) showed very powerful suppressive effects of mouse immune cell death and significant activities of cell proliferation in vitro. Especially, the methanolic extract of Rosa rugosa was found to have fifteen times compared to the control treatment, demonstrating that Rosa rugosa may have a potent stimulation effect on immune cell proliferation. These results suggest that several salt marsh plants including Rosa rugosa could be useful for further study as an immunomodulating agent.