Desmosterol (1), 24,25-epoxycholesterol (2), 24-hydroperoxycholesta-5,
25-dien-3 beta-ol (3), 25-hydroperoxycholesta-5,23(E)-dien-3 beta-ol (
4), cholesta-5,25-diene-3 beta,24-diol (5), and 24,25-epoxy-6 beta-hyd
roxycholest-4-en-3-one (7) were isolated from the marine red alga Gala
xaura marginata; sterols 3, 4, and 7 were isolated for the first time
from a natural source. Sterols 3-7 exhibited significant cytotoxicity
toward serveral cancer cell lines.