The diagnosis and classification of leukaemic diseases is still primarily based on morphological and cytochemical criteria. Interpretational difficulties occur quite frequently, especially in acute leukaemias. Subjective morphological cell-type characterization--which is acquired only after many years of experience--is rather unsatisfactory in the long run. Therefore in order to obtain uniform results in all haematological areas, new methods are needed. In principle, immunological cell-type characterization represents an alternative method. By applying requisite antibodies practically any cell component can be demonstrated and quantitated. Up to recently the major obstacle to a more rapid development of immunological cell typing was the lack of specific antisera, but with the introduction of hybridoma technology it became possible to obtain monoclonal antibodies and thus, markedly improve immunological cell typing virtually overnight. We have, therefore, used this new technology for the production of monoclonal antibodies to human leucocyte antigens. This paper describes six of the cell-type specific monoclonal antibodies obtained, their suitability for diagnostic purposes and the classification of human leukaemias. With the help of these antibodies the majority of human leukaemias can now be typed. Without claiming completeness and in full awareness of the fact that a number of problems remain to be solved, we nevertheless believe that the presented data point to the possibilities opened up by this technology for leukaemia diagnosis in the future.