The gorgonian coral Briareum asbestinum (Pallas) (Briareidae) occurring in Caribbean waters contains diterpenes of two skeletal types, the asbestinins and the briareins, some of which are endowed with biological activity. In the course of searching for biologically active compounds from marine sources, a specimen of B. asbestinum collected near Sandy Cay, Bahamas, at a depth of 2 m was investigated. Extraction afforded a new metabolite named brianthein V [5] together with briantheins Y and Z. The structure elucidation of 5 and the biological activity of these compounds are reported. Workup via solvent partitioning, flash chromatography on Si gel, and HPLC on a C18 column yielded pure diterpenes 3-5. Spectral data confirmed 3 and 4 as briantheins Y and Z. Compound 5 has the molecular formula C30H41ClO10 (extra C2H4 fragment vs 3), indicating a butyrate instead of an acetate group vs 3. X-ray analysis of 5 (crystallized from MeOH) provided its absolute structure, which has ring conformations similar to briarein A and brianthein Y, with the cyclodecene ring adopting a conformation like brianthein Y, the six-membered ring having a C2 conformation, the trans-fused γ-lactone as an envelope conformation, and crystallizing with one MeOH molecule via hydrogen bonding. Biological screening showed diterpenes 4 and 5 are cytotoxic (in vitro P388 assay: 4 at 10 µg/ml, 5 at 13 µg/ml). Compounds 3, 4, and 5 display antiviral activity: in the in vitro mouse corona-virus assay, inhibitions were at 400, 80, and 50 µg/ml respectively; compound 4 is active against herpes simplex-1 virus at 80 µg/ml in vitro.