This volume is the collection of papers presented during a four day meeting, the EMBO workshop "Protein Conformation as an Immunological Signal" that took place at Portovenere (La Spezia), Italy, October 1-4, 1981. The motivation that drove us to organize this meeting was the feeling that distinct groups of researchers, active in key areas of modern immunology, sometimes fail to communicate with each other simply because of different traditional affiliations. Yet it is urgent that "molecular" and "cellular" people cooperate more if immunology is to continue the exportation of new concepts to other disciplines. In fact, the deep meaning of molecule-molecule and cell-cell interaction, the generation of signals and their effective transmission which results in elicitation, control or suppression of responses cannot be unraveled without the experts on antibody structure or complement activation sharing their views with the experts on T cell, B cell and macrophage membrane receptors as well as the experts on factors that carry the information released by these cells. Whether the meeting was scientifically fruitful, the reader can judge after having digested these pages. We, the organizers, are not sure whether the optimal amount·of interaction had taken place; especially considering how hard it is to overcome the scientist's catch 22: You have to know something quite well before you get really interested in it. In any event, we are convinced that Portovenere was one of the most successful attempts we have witnessed.