The Concerted Action (CA) was formed to determine the processes which lead to the entry of the human immunodeficiency virus (HIV) into the target cell and those involved in the formation and release of HIV in the target cell. Research is planned to obtain a detailed insight into the molecular mechnisms involved in virus attachment to the receptor molecule, the conformational changes leading to the fusion of viral proteins with the cell membrane as well the molecular mechanisms necessary for the processing of viral proteins leading to particle formation and interaction with the cell membrane in virus assembly and budding. The ultimate aim is to provide a solid basis for the design of new strategies to prevent infection and to interfere with the production of viral progeny for prevention and therapy. The CA has attracted leading European scientists from several appropriate fields. The transfer of information has been organized in the form of laboratory exchanges, small meetings focused on certain problems and annual meetings. These practical exchanges are supported by a centralized facility producing high quality peptides and monoclonal antibodies.This programme is of basic importance for other fields in HIV research as its results may influence the strategies of several other projects involved in vaccination, therapy development and pathogenesis of HIV infections.