This is the first report on the antitumor and immunostimulating activities of Ulva lactuca. Using the WSM (water-soluble fraction of a methanol extract from Ulva lactuca), a concentration of 140 g/mL was found to inhibit 50% of the human leukemia cell line U937 in growth, while splenocyte growth was stimulated up to a concentration of 100$mu extrm{g}$/mL. In addition, NO production by a macrophage cell line (RAW 264.7) and alkaline phosphatase activity in mouse splenocytes were both stimulated with 10$mu extrm{g}$/mL of WSM. Dose-dependent patterns were observed on all three cell-lines. Accordingly, the current results indicate that VUlva lactuca may be useful as a natural antitumor and immunostimulating agent.