In this document, the known and suspected effects of various classes of contaminants on marine mammals are reviewed. The document then considers in more detail the evidence linking organochiorine contamination with reproductive and immunological disruption in marine mammals, evaluates the problems in interpreting this evidence and suggests what further work is necessary to clarify the situation. The document speculates about future changes in environmental levels of key contaminants and the effects these may have on marine mammals. It then makes suggestions about procedures for the collections and analysis of material from marine mammals. Finally, some recommendations are made for further research and management action.