New methods helpful in the classification of lymphoid and monocytoid leukaemias and haematosarcomas have recently become available. Among the most valuable procedures have been those detecting immunological markers and the ultrastructural analysis permitted by convention and scanning electron microscopy. The results of these studies, taken together with a meticulous morphologic examination on Giemsa smears allow a more subtle classification on these neoplasias. Among the most interesting points, one can list: a) The observation that all the prognostic parameters in the four types of common acute lymphoid leukaemia (ALL) (prolymphocytic, microlymphoblastic, macrolymphoblastic, prolymphoblastic) such as age, tumour mass (or leucocytosis) are related to the cytological type; T marker may be found in all these types, except in the prolymphoblastic; b) T or B acute lymphoid immunoblastic leukaemias has been described as a fifth type of ALL; c) The lymphosarcomas which can be histologically nodular (composed of B cells) or diffuse (composed of cells presenting B, T or no markers) can be cytologically prolymphocytic or lymphoblastic (or lymphoblastoid) if they are nodular, prolymphocytic, lymphoblastic or immunoblastic if they are diffuse; true (African) Burkitt's lymphosarcomas (BLS) must be distinguished from all the pseudo (non-African) Burkitt's lymphosarcomas which have been described elsewhere and which are either immunoblastic or lymphoblastic (oid) lymphosarcomas with a few macrophages. d) The diagnosis of reticulosarcoma which was in the past abusively carried out (because of confusion with immunoblastic lymphosarcoma) is based on the presence of many reticulins fibers at histological examination and on the cytological aspect of the cells on smears (the distinction of those cells with immunoblasts is easy).