A total number of 31 types of seaweed were assessed with regard to their effects on the proliferation of mouse spleen and thymus cells in a culture, using an MTT reduction assay. Acetone:dichloromethane (1:1) extracts of three seaweed plants: Derbesia marina, Sargassum sp., and Hisikia fuziformis, exhibited significantly positive effects on the survival of mouse spleen and thymus cells in vitro. The acetone:dichloromethane (1:1) extracts of Sargassum sp., in particular, much more potent effects on thymus cell activation than did any of the other types of seaweed. However, the methanol extracts of Sargassum ringgoldianium and Chondrus crispus exerted a stimulatory influence only on the proliferation of mouse spleen cells, whereas the methanol extracts of Grateloupia lanceolata exhibited significant cell proliferation properties in both spleen and thymus cells.