One of the most rewarding applications of anti-leucocyte monoclonal antibodies is their use in the study of leukaemias and haematosarcomas. They are now of daily assistance for the diagnosis and classification of these diseases. The availability of malignant cell phenotypes provides a more accurate definition of nosological entities and opens new prospects of a better adaptation of treatment to each individual patient. Moreover, monoclonal antibodies now make it possible to determine the general characteristics of the phenotype of malignant leucocyte populations. Such an improvement in our understanding of these tumours will undoubtedly result in numerous developments during the forthcoming years.