Seaweeds contain a variety of biologically active substances, which are useful for therapy and medicinal purposes. One of the potent uses has been found in its immune functions. In the present report, we describe immunological properties of the marine brown alga Endarachne binghamiae against murine macrophage and human T cells in vitro. It was observed that various extracts from the alga effectively stimulated cell proliferation and that the stimulation activity of active substances varied with growth habitat of the alga assayed. The reported alga was found rich in polysaccharides. Of which sodium alginate exhibited strong stimulation activity for macrophage and T cell proliferation, and also alginic acid but to a lesser extent. A glycoprotein isolated from the reported alga was also a strong proliferation stimulant. Additionally, it significantly induced the production of TNF-αand nitric oxide by macrophages and IFN-γby T cells in a concentration-dependent manner. These assay results suggested that alginate and protein of the reported alga could be promising immune stimulants and modulants.