Chemical investigation on a cultured octocoral identified as Briareum sp has led to the isolation of a new 12-hydroperoxybriarane, briarenolide D (1) and a known briarane, 2 beta-acetoxy-2-(debutyryloxy)stecholide E (2) The structure of briarane 1 was established by spectroscopic methods and the structure of 2 was further confirmed by X-ray data analysis for the first time Cytotoxicity of hydroperoxybriarane 1 toward various tumor cell lines is also described