At the request of the Leukaemia and Haematosarcoma Reference Centre of the World Health Organisation (W.H.O.), one of the authors (G.M.) has suggested a classification taking into account the most recent knowledge and all parameters (histological, cytological and immunological) necessary for the categorisation of non-Hodgkin's haematosarcomas. He has employed traditional terminology, thereby avoiding the jargon used by certain authors which is not justified in the light of modern scientific knowledge which, more than ever, merits the use of classical terminology. The authors present this W.H.O. classification for the first time to the french press. The reader will find familiar terms such as "lymphosarcoma", which may take a nodular or diffuse form (in the second case the recently identified immunoblastic form is introduced and mycosis fungoides is placed in the category) and reticulosarcoma. Microscopic typing of a haematosarcoma is essential since the various types differe considerably in their sensitivity to treatment with a corresponding difference in the chance of cure. The potential for cure is also dependent upon the topographic stage which should be determined by a series of routine step which may still be recommended today but which may be useless in the future by virtue of the frequency of disseminated forms in relation to localized forms, and of the remarkable effectiveness and constant progress of chemotherapy and immunotherapy.