Over a small number of years many monoclonal antibodies directed against leucocytes have been produced. Their multiplicity and the various nomenclatures utilized may be disconcerting. As a result of the First International Workshop on Human Leucocyte Differentiation Antigens (Paris, 1982), a very clear correspondence between these different antibodies has been established by regrouping into "differentiation classes", each class corresponding to a given molecular species at leucocyte surface. One of the consequences of this international co-operative study has been the development of a nomenclature system now officially adopted by the World Health Organization. In addition, it is now possible to identify the general characteristics of leucocyte surface molecules, which provides information on their organization and function. The international study is in progress, and the results of the Second Workshop (Boston, 1984) will soon be available.