From the Antarctic sponge Lyssodendoryx flabellata, a new polycyclic compound, which we named flabellone, related to the oestrogenic hormone oestrone, has been elucidated by spectrometric and spectroscopic means. Along with flabellone, a glycosphingolipid (GSL) mixture featuring an unusual α-fucofuranosyl-3-β-glucopyranoside unit structurally identical to the previously reported terpioside has been isolated and identified. The mixture of GSL homologues exhibited inhibitory effects in mixed lymphocyte reactions on human cells.