Styela clava (SC), a sea squirt has a biphasic life cycle, showed the animal phase during larval life and the vegetable phase during adult life. The adult SC becomes fixed on hard surface and covered with thick cellulose membrane to protect itself from various physical and chemical damages in marine environment. Especially, adult SC forms a huge cystic space as a portion of digestive tract, filled with seawater containing various planktons. In the present study, it is hypothesized that SC can produce secretory materials, like saliva in mammals, to sterilize, to neutralize, and to digest the cystic fluid. First of all, the cyst fluid was examined for the antimicrobial property by bacteria killing assay (BKA) using E. coli, and it resulted that the cyst fluid showed relatively weak bactericidal effect. The cyst fluid was also examined for antioxidant property by DPPH (1,1-diphenyl-2-picrylhydrazyl) and ABTS (2,2-azinobis-3-ethybenzthiazdine-6-sulphonic acid) assay, and resulted that the cyst fluid showed strong antioxidant effect comparable to ascorbic acid. The antioxidant materials of cyst fluid were found both in cationic and anionic groups separated by ion exchange column using SP and Q beads, and they were also heat resistant. The strong antioxidant property seems to be originated from the abundant carotenoid materials in the cyst fluid of SC. However, the strong antioxidant materials of the cyst fluid may play a role for the essential antioxidant in the digestive tract of SC. Furthermore, the cyst fluid of SC was examined using SNU-1 cancer cells for its anticancer property. The flow cytometry analysis showed that both of cationic and anionic materials purified from the cyst fluid showed strong anticancer effect on SNU-1 cells. Taken together, the cyst fluid of SC showed weak antimicrobial, but strong antioxidant property and marked anticancer properties in vitro experiment. Therefore, it is presumed that SCs can neutralize the various free radicals of seawater and also protect themselves from carcinogenic impacts in aggressive marine environment by various bioactve substances of their cyst fluid.