An investigation of the marine sponge Halisarca sp. was carried out following the discovery of strong activities in antiviral and P388 murine leukaemia assays of an extract from this sponge. A polyether type of compound is suspected as the cause of this activity. The investigation of Halisarca sp. made a significant contribution to the development of a chemical screening process. The absolute configuration of pateamine has been elucidated by a combination of degradative and synthetic chemistry utilising formation of diastereoisomers with Mosher's acid. Several other derivatives of pateamine are reported. The algal metabolite thyrsiferol has been isolated from the digestive gland of the marine mollusc Aplysia parvula. The egg masses of A. parvula have also been examined. The NMR characterisation of the thyrsiferol has been extended through the extensive use of one- and two- dimensional spectroscopic experiments.