Masters of Science / A chemical study on the total extract of the zoanthid Palythoa tuberculosa, collected from the Red Sea, resulted in the isolation of seven polyhydroxylated sterols viz: palysterols A-G, six of which are new. Their chemical structures were elucidated on the basis of their 1D and 2D NMR and MS spectroscopic data. Palysterols B and G demonstrated cytotoxic activity on three human cancer cell lines (MCF-7, HeLa, and HT- 29). Palysterol G, in particular, was able to induce apoptosis in breast adenocarcinoma(MCF-7) cells